Camping is a legitimate strategy. The problem occurs when a game's levels or systems aren't designed to give the opposing side a way to deal with a camping player. Several MW2 maps come to mind.

I still love this old Penn Arcade comic on camping: http://www.penny-arcade.com/comic/2002/4/22/

I think Donkey Kong was the original camper. Just stayed up there throwing barrels like a pussy.