Someone on brickshelf designed a bumper which presses the NXT buttons when activated - thus freeing up two sensor ports and touch sensors (or whatever sensors that would be used to detect an object).
View the other pictures here
Looking at it closely, I can see it's a pretty ingenius design if it works: there are two extensions on it - one in front and one in back - that press one of the arrow buttons when pushed down (each extension is attached to an axle which can rotate, lifting or lowering the blue pegs, thereby releasing or pressing the buttons). They were positioned in the front and back of the robot such that, when something bumped into them head on, they would be pushed down, and press one of the arrows.
I know some people have been talking about making one of these (mainly for FLL to get extra touch sensors), but this is the first time I've seen one actually built - and a really good one at that!