There have already been sightings of screenshots proving a FaceTime app for iPod Touch, and presumably all iDevices, is in the works. Looks like our friends at 9to5mac have got their hands on even more info about exactly how FaceTime will work on non-iPhone iDevices.
As previously speculated, it looks like you will use your Apple ID in order to get a username and password for FaceTime, just like GameCenter in IOS 4. This is obviously to get around the fact that the service cannot run through a phone plan like on the iPhone, since other iDevices do not have phones. You will also be able to choose a username to show up as a caller ID when making FaceTime calls from your iPod touch.
9to5mac is reporting that FaceTime will be available on iPods this Fall, and the image above is a screenshot of what the FaceTime settings app icon will apparently look like when its implemented.