Tablet Talk: the OS question
If you're creating a tablet computing device, you'll need an operating system for it. Unless you want to create an entirely new OS, you've really got two choices: adapt a desktop OS (Windows, Mac OS X, Linux), or use a smartphone OS (iPhone OS, Android, etc.). Neither of these options was designed for a 7-10" device meant to be operated primarily by touch, and they bring different challenges to that particular application.
The desktops OSs are extremely powerful and flexible, but they've evolved in response to a particular input method, namely the keyboard-mouse combination. While it's possible to scale the display of the OS in order to be able to interpret it visually on a small screen (there are plenty of 9-11" Windows netbooks, for example), navigating the UI is another story: imagine trying to click the "close" button on a window with your fingertip without accidentally clicking one of the adjacent buttons, and you'll get a sense of the problem. One traditional answer to this problem has been the use of a stylus, which immediately poses its own problems: you're forced to hold the device with one hand and the stylus in the other, or cradle the device in the crook of one arm, and if you lose the stylus, you're hurting. Another answer has been to make convertible notebook tablets, which appear to be regular laptops but allow you to rotate the screen and fold in back overtop the keyboard to use the device in tablet mode; you can then select the mode that makes the most sense for your particular application. This means, however, that you're carrying that keyboard around whether you use it or not, and the subsequent weight and additional thickness significantly impacts the ergonomics of the device. I'd argue, actually, that the convertible tablet is not actually a handheld device: it seems best suited for notetaking on the screen when you have the luxury of a surface on which to rest the tablet.
Both of these types of tablets are currently available: slate-type "UMPC" devices have been in production since Microsoft, Intel, and some partners announced Project Origami in 2006, and convertible notebook designs are older than that, and still in production by HP among others. Never heard of them? You're not the only one. I've seen a convertible notebook at Best Buy, but never in the wild, and I've only seen the slate UMPCs advertised and discussed on the Web. They seem to have gained some limited popularity with certain mobile users, and with some specialized industries (apparently convertibles are more common in the medical field), but they're far from mass-market items. While tech geeks keep insisting that tablets running a full desktop OS are the only way to go, it seems to me that the market has spoken against that particular option.
The other direction to go with a tablet OS is to use one of the mobile device operating systems that currently run smartphones. Given that Apple's iPhone OS is proprietary, most of the devices that go this route use the open-source Android OS championed by Google, and recently made more visible through the high-profile launches of Verizon's Droid line and the Google Nexus One. These operating systems have the advantage that they're optimized to run on devices which operate primarily or totally on touchscreens for input, and they're lightweight enough to run quickly on the lower-power hardware that's preferred for mobile devices. There are a few Android tablets currently available - most notably, perhaps, the Archos 5, which you can get at Best Buy and Radio Shack - and a number of others are scheduled for release this year. Many of the latter are using NVIDIA's new Tegra 250 chip, the successor of the processor found in the Zune HD, which boasts stellar increases in video rendering performance for games and HD video. I'd argue, in fact, that the Android-based Notion Ink Adam is probably the most imaginative piece of tablet hardware that has yet been announced. The iPad, of course, runs the iPhone OS - or, more precisely, a variant of that OS; we'll get to that point in a minute.
Android tablets suffer from a couple of notable disadvantages. First, the OS has mainly been developed to operate devices with 3-5" screens - specifically smartphones. Scaling up to a 7-10" screen means that UI choices made to accomodate the smaller screen size may no longer make sense, and may in fact impede the user experience. Secondly, there's a fairly small universe of applications available for Android devices (about 20,000 as of this writing) compared to the iPhone app ecosystem, let alone the Windows software universe. And several of the most-desirable apps for that platform, including the GMail app (the *only* native app for GMail available for any mobile device) and the Android Marketplace, are proprietary Google apps which are only available on Google-approved devices, which to date has meant smartphones. The Archos 5, for example, only has access to the Marketplace if it's hacked, and has suffered substantially in my view from that lack; the enTourage eDGe that I considered buying doesn't have access either. Granted, Google could choose to make its apps more broadly available, and the ability to sideload applications on Android devices means that alternative distribution channels could be (and have been, to some extent) created, but at the moment you can't get the same Android experience on a tablet that you can get on a smartphone. [EDIT 2/19/10: Engadget's review today of a prototype of Dell's Mini 5 Android tablet indicates that it has access to the Android Marketplace, so maybe Google is shifting its position on this issue.]
Given these two alternatives, Apple went for the second, giving the iPad a version of the iPhone OS instead of OS X (and bitterly disappointing a large portion of the tech geek crowd, who desperately wanted a MacBook tablet). However, rather than simply porting the OS to the iPad, they made some significant changes to it, retaining compatibility with iPhone applications while adding UI elements that make much more sense for the increased screen size, like pop-over menus. Hopefully the folks making Android tablets will take some hints from this approach for their own systems - I think Android's got a lot of potential as a platform, and Apple needs some competition in this sector to push it to keep innovating.
[EDIT: an important item I forgot to mention last night was that desktop OSs have also been optimized for power-hungry processors; the lightweight chips that run mobile devices struggle with or are outright unable to handle computing-intensive desktop apps (complex games, Photoshop, video editing, and often even HD video playback). So while it may appear that a Windows tablet has much more available power, in reality the user is limited to a low-intensity subset of those applications.]