May guess is that with tablet we will be using the same App Store and just like with iPhone OS 3.0 new apps will be tablet compatible (fullscreen mode) with two modes one for iPhone and one for tablet. If you download that app on iPhone that app will run in iPhone mode, if on tablet - tablet mode. And older apps will run like widgets. But with Apple you never know until they smack you with pure brilliance in a way you would never think of.
I think apps will have to set a BOOL in their info.plist that states they are tablet-compliant, otherwise they are run in "iPhone mode" (either windowed or scaled to fill the screen, not sure which). If the BOOl is set, then the app runs full screen/full resolution.