I did the Blockbuster thing for my copy of Lost Planet, the younger cousin got hold of it and it got scratched, completely unreadable, rented it out, swapped over and all was ok.
My mate has tried it but they checked the disc before he took it and and upon return got served by the same person and she knew the state of the disc when he took it out and wouldnt accept it!
They do have "rental version" copies too, which in my eyes are no way different to retail purchased ones, just have different coding on them i think, not too sure, but ive seen them about lately.
Gears of War FTW!