I wouldn't be surprised to see this one hit the projected release date fairly accurately. Here's a couple of reasons:
1) The transition from XP to Vista involved a near total rewrite of the code. Vista wasn't an incremental update, it was a full replacement. Windows 7 will be based on Vista, therefore alot of the code is already written.
2) MS received tremendous amounts of flack for botching the release of Vista so completely and so often. I don't think they will be willing to make the same mistake again so soon...even if it means leaving out features that aren't ready to ship when the time comes.
It could be an expansion pack for Vista Like we have expansion for games I wouldn't be suprised if its sold like that. It could be sp3 for vista.