There's over 100 hours of gameplay to begin with. I don't see why everyone cares so much about DLC. It most likely (and by most likely I mean 99% likely) won't even be free, so unless it can add a significant amount of gameplay, why bother?
Also, to Coaster Individual, I'm not sure exactly what your point was in your post, but the point of using Blu-Ray isn't necessarily to utilize the whole 50gb; it's to expand the limitations. There's only so much you can put on a DVD, and I'd be surprised if Rockstar hasn't had a significant amount of trouble getting Liberty City to fit on one.