In Borderlands, what if my character is past level 50 and then I play without the DLC installed?

I can't say for sure, but I would guess it won't allow you to play.

Character information is most likely stored in the game save, which was probably altered somehow by the DLC, so I guess a game without the DLC won't be able to read it properly.

I would test it for a more "official" answer, but I don't own the game.