We've already seen rumors that FaceTime would be tied into Apple or iTunes IDs, and if Apple does indeed plan to roll out the service to future iPod touch and iPad devices with front-facing cameras but without phone numbers, it's a logical and necessary step.
Why Apple didn't use email as the basis for iPhone 4 FaceTime to start with is anyone's guess but the idea of an Apple supported VoIP system for both video and audio (hit the home button to exit the video but keep the call going), that's standards based and released as an open standard, is certainly interesting. If every iPod touch 4 and iPad 2 supports it, and other apps and devices running Android, webOS, etc. pick it up, it could be even more interesting.
Until/unless it supports SkypeIn/SkypeOut-like ability to call people on regular old phones it won't be a threat to traditional telephony, but it could be a strong contender to the future.
I'm already using FaceTime whenever possible. I'd like to use it more. Opening it up to connections based on email is the first step.
Hopefully we'll learn more at Apple's traditional fall iPod and iTunes event, where iOS 4.1 is widely expected to debut.
[MacRumors, thanks Tristan!]