Does the PC (steam) version of ME2 support the gamepad?  I haven't been able to find a definite answer to this anywhere.

No

No version supports  gamepads. Give the mouse and keyboard controls a shot, I actually prefer them to gamepad for Mass Effect 2.

I don't think it does, since the retail version doesn't and it seems unlikely they would enable it just for the Steam version, especially without releasing a patch for the other version.  
I'm not 100% sure though.

" Does the PC (steam) version of ME2 support the gamepad?  I haven't been able to find a definite answer to this anywhere. "

Officially, no, but there was a mod thing that unlocked the a lot of the 360's UI, but with many glitches like low sensitivity, partially invisible power wheel, problematic use of Left Stick in the Pause Menu, and you can't start a new game with a gamepad.
 
In other words, don't bother.

Thanks for the quick responses all.

I've tried using both Pinnacle and Xpadder and while the game definitely playable, the mouse/keyboard controls are already superior to the gamepad controls on the 360 so it's kind of pointless for them to put in gamepad support.

@spazmaster666: That's an opinion.  I'd say Dragon Age is clearly more suited for mouse/keyboard and the game benefits greatly from it.  But Mass Effect was clearly designed as a console game and in my opinion works a lot better with the 360 pad.