Apple appears to be doing with FCP X what they did with Aperture when they put that in the App Store: reducing the new purchase price to the old upgrade price, and eliminating upgrade pricing entirely.
So, no, you won't need to buy or install FCP 7. FCP X will just be $299 in the App Store, regardless of what version you're coming from or whether you previously bought Final Cut at all.
Apple were entirely specific about this in the presentation: No upgrade pricing, just a single purchase price for anyone of $299.
This is another reason why I don't get the pricing debate (aka the ridiculous "it's too cheap" argument), as not only is the price for a single app, but even if it's not (which I secretly suspect it isn't - there's more to come at this price) most people will be upgrading, and would only have paid $299 or very similar to update. There's 2m users after all...
Edit Out Ltd
FCP Editor/Trainer/System Consultant
EVS/VT Supervisor for live broadcast
RED camera transfer/post