Actually, from what I can see, Windows 7 is to Vista what Windows 98 Second Edition was to Windows 98. Yes, there are a few added features, but for the most part it is Vista revisited.
And they are not releasing it as SP2 because they want to make money - and historically, service packs have been released free of charge. Since Vista sales have not been what MS wanted them to be, they are trying to make up for that with a name change - but I don't see anything that distinguishes "7" from Vista other than the name.
I think that anyone with a Vista license should be able to plug their vista key into "7" and get activated instantly. It won't happen, but it would be the right thing for MS to do.
At the very least, they could offer "7" as a Vista upgrade for 10 or 20 dollars, as they did with Second Edition (for 98 users).