There are rumors about the next iPod touch getting a front-facing camera with support for FaceTime. Once it gets an A4 processor and some more RAM, we can't see why it wouldn't.
Since there's no phone app, however, how will users initiate FaceTime calls from an iPod touch? According to Boy Genius Report (BGR), the FaceTime app will look for a registered email address.
Paraphrasing an anonymous source, BGR claims that users will register their Apple IDs (typically a MobileMe email address) on a given device. That way, iPod touch owners can initiate a FaceTime call via this registered address. Push notifications will inform you of an incoming FaceTime call.
It's still unclear how an iPod touch will initiate a call with an iPhone, as that currently requires the iPhone's phone number. BGR speculates that the registered email address method will be carried over to all devices. iPods are typically refreshed in September, so we'll see if anything about this surfaces before then.