Regarding the game world size, I've also wanted a bigger size than the actual screen size to layout everything. I think it can be done, however you can't do it through interface builder because that only allows editing within the iphone screen.
For collisions you can use CGIntersectsRect(character,building){DO SOMETHING} which will then do something when the two images "interesect" or touch essentially. And yes, if you have a background image, you'd need a separate image saved of just the building so that the game can detect where the building is.
For alternate views, you could have two views, and then using CGIntersectsRect(character,door){load second view containing building interior}
Hope that helps
