My own little shootout.
ps3
Hardware
Features: A (20g and 60g models), B (newer 40g and 80g models)
Reliability: A
Peripherals: C
PS3 hardware is fully featured, with an internal hard drive, blu-ray, wireless networking and controllers, memory card slots (select models). Backward compatibility was lost on newer models. Most powerful hardware out there, though the software hasn't shown it to date.
Software
Current Game Library: D
Future Game Outlook: B
The current game library is rather meager, with most multiplatform games playing much better on the 360. Exclusives are limited this year, but more will come next year including God of War, Gran Turismo.
Online
Features: C
Reliability: C
Online is free, which is a plus, but it's nowhere near as fully featured as Xbox Live, though it handily beats Nintendo's offerings.
Wii
Hardware
Features: B
Reliability: A
Peripherals: B
The Wii hardware is small, reliable, and backward compatible with Gamecube. It also features wireless networking and controllers. It's the quietest console of the three. Looks like it was designed by Apple.
Software
Current Game Library: B-
Future Game Outlook: B
The current software library is mostly minigames, however there are quite a few AAA titles including Zelda, Metroid, and Super Mario Galaxy. The future outlook is also strong, as quite a few developers switched resources to the Wii after it's success at retail, and those games should be hitting shelves shortly.
Online
Features: D-
Reliability: C
If you want to play online, look elsewhere.
360
Hardware
Features: D
Reliability: F
Peripherals: B+
Very vanilla, no wireless networking, regular DVD drive, very loud, ridiculous power brick and terrible reliability. The only bright spot is the well designed controller. To their credit, the warranty has been extended to 3 years. The problem is, you'll probably need it and two years after the console's launch the problems still aren't fixed.
Software
Current Game Library: A-
Future Game Outlook: B
Strongest game library of the three by far, with many AAA titles like Halo3, Gears of War, BioShock, Mass Effect. Multiplatform games usually have the best version on the 360 as well. Next year looks great too. Only weak spot is lack of support from Japan.
Online
Features: A
Reliability: A-
Best online experience by far with cross game messaging and chat and best UI. But it's also the only pay service of the three. Some weird echo issues sometimes.