Personal Document Archiving
An important change for those of us who use the Kindle's wireless capabilities to send fanfic (or other personal documents) to our Kindles. This change only applies to the Kindle 3 (aka Kindle Keyboard), the new $79 Kindle (nickname needed), and the upcoming Kindle Touch.
A feature in the next software update (version 3.3; it's not available yet for most users, but probably will be soon) is Personal Document Archiving. Like any book you buy from Amazon, personal documents sent wirelessly (apparently either 3G or wifi) to the Kindle will be archived by default on your Manage Your Kindle page. Although this will be officially part of 3.3, some users are reporting that documents sent to themselves are now showing up in the Personal Documents Archive.
The upside is that you will be able to download any deleted document from your Archive wirelessly later on, even after you delete it from your device, just like a purchased Amazon book. Individual documents, just like books, can be deleted from your MYK page. The total limit is 5 gigs, which, hey, can hold a heckuva lotta fanfic, if this is something you're comfortable with.
The potential downside is that your personal documents will be backed up on Amazon's servers, and viewable on your MYK page by anyone with access to your account, which hasn't previously been the case with personal documents. The good news is that you can completely opt out of this service. If you don't want Amazon backing up your personal documents, go to your Manage Your Kindle page, choose "Personal Document Settings" from the left-side bar, and under Personal Document Archiving, select "Edit" and then untick the check box.
As far as I can tell, Personal Document Archiving applies only to documents sent wirelessly to Kindle devices, not to documents transferred via USB or downloaded directly from a website through the Kindle browser. More about this should become clear over the next few weeks and as the 3.3 update starts officially rolling out.