First off, obviously windows 7 is not the final windows, so considering (IMO) Microsoft will move forward , there will be a successor.
Next it is common practice to release a new OS every 3-7 years (a operating systems expected lifetime) which puts the timeline into perspective, 7 was only announced so rapidly because of the many mistakes in Vista (and it's bad rep) for marketing reasons it made sense to market 7 sooner then later
Finally, all my research leads me to believe that concept of 8 is real because it was mentioned at the PDC plus (supposedly) in a KB article that was later edited to remove it's reference.
Lets consider this, from a marketing standpoint, it is better to start rumors about a New product way before it comes out, let people debate it, then release it.
Look at the I-Phone with IOS4, there was a debate going back and forth for a long time as to it's authenticity , by the time they actually announced it, everyone wanted one, the reason is "status" it was surrounded by so much hype, they sold off the shelves.
One of the 4 P's of marketing is promotion, almost all good Viral campaigns start with a rumor a misleading controversial article, or some "X Employee" who uploads video concept ideas.
So yes, in short, I can say that if nothing else, the concept of the "Code Named" Windows 8 is probably a very real product, and I look forward to it.