Obama wins by lots and none at all.
I wouldn't be at all surprised if the Electoral College vote shows a landslide for Obama but the popular vote is within just a few points. I think Obama wins enough swing states by very little to push him over the top, but we won't know it until the wee hours of Wednesday morning.
Obama by 10+: VT, MA, RI, CT, NY, NJ, MD, DE, DC, IL, OR, HI
Obama by 5-10: MI, WI, WA, CA
Obama by 0-5: NH, PA, VA, OH, MN, IA, NM, NV
McCain by 0-5: NC, GA, FL, IN, MO, CO
McCain by 5-10: WV, AR, LA, KS, ND, MT
McCain by 10+: SC, KY, TN, AL, MS, AR, SD, NE, OK, TX, ID, WY, UT, AZ, AK
That's Obama 302 - 236 McCain.
I'll go further and say that VA, OH, MO are within 2 points. Maybe even NC thanks to Elizabeth Dole's self-immolation. Bob Barr makes things interesting in a few states like GA and NH but doesn't cost McCain the election the way Ralph Nader held back Gore in 2000.
While McCain racks up individual states, Obama wins the population centers and EV-rich states like NY and IL by huge margins, propelling him to a 52-46 win in the popular vote. Not exactly a mandate, but a far more convincing win than Bush in 2004.
I think Palin holds onto the GOP's evangelical base in CO, ND, GA, and NC. Obama can't win over all the old people and Cubans in FL. Obama wins by a comfortable 5 points in PA, IA, and MN, and McCain by a similar margin in IN and CO.
Regardless, the map is radically changed from 2000 and 2004. More states are more "purple" than before, even down to the county level.
As for the downticket races, I don't see the Dems getting the magical, mystical 60 votes in the Senate (including two independents). I think Stevens goes down in AK (though he gets within single digits) and Dole falls in NC. Coleman in MN and McConnell in KY hang on. Without looking at each individual race, I'll give them fifty-si-seven. And throw in around 20 seats in the House, including holding onto GA-8 (Marshall).
Georgia's Saxby Chambliss will be forced into a runoff with only 48.5% of the vote but wins the ugliest election in Georgia history by 7 points in December. It's going to be disgusting on all sides. I plan on putting down tarps in my living room to protect from all the mud being slung when I turn on the television.
Overall, it's a big day for the Democrats and a huge vindication of Howard Dean's 50 State Strategy. Dean and Obama solidify their hold on the party, and while Hillary Clinton remains a force to be reckoned with, her cronies (*cough* Terry McAuliffe *cough*) are on the outside looking in. The question will be in 2014 to see if the ground game created this year (and carried over into the 2012 campaign, win or lose) will still be available for the 2016 candidates.
I have no idea what happens to the GOP. There will be a huge power struggle between the religious base (led by Sarah Palin?) and those more interested in fiscal and foreign affairs (led by Ed Rendell/Mitt Romney?). The winner will blame the other for losing this year. Those who follow James Dobson and his ilk will claim that McCain lost the soul of the party, and for the GOP to regain power, it will have to follow the road that put them in power in the first place, built by the evangelical base. The George Wills of the party will claim that putting Palin on the ticket cost the party the middle ground and the 30% of the voters who are moderate/independent.
With any luck, the theocrats break off and form their own party. The Dems stay in control at least through Obama's two terms, and then they fall apart at their own seams. Maybe by 2020 we'll have four or five viable parties in this country. That would be awesome.