It looks like you’re stuck with the mouse and keyboard if you’re looking to play the newest adventures of Commander Sheppard on your PC, since Mass Effect 3 won’t be supporting anything else.
This is not to say that plugging in an Xbox 360-styled controller into a USB port won’t work, just that EA doesn’t promise that it will. Chris Priestly announced the lack of support to the fans on the Bioware forums a few hours ago. A fair number of ported-to-PC games don’t support additional controllers, including Mass Effect 1 and 2, so it’s not exactly a huge surprise.
Mass Effect 3 comes out on March 6th in the US and March 9th in the UK. Have you checked out the new demo released yesterday yet?