We don't really have anything special for a cart; just a c-channel frame with 1" wood inside. It's great because we can store stuff on the bottom, we have wagon wheels for crossing over the various terrain of FIRST, and with the wooden top we can screw on whatever type of robot holder we may have.
It's also ridiculously lite.
Did I also mention it can also move people?
http://www.flickr.com/photos/team201/4364054426/
Our old cart was made out of heavy steel with a shopping cart handle. It was only one level, but you can fit 2 robots on it at a time.
http://i255.photobucket.com/albums/h...5team_dpn4.png
In my opinion the most important things about a cart is that
a) Big Wheels
b) Can Store Tools
c) Has 4 Casters on it
Casters are extremely useful for navigating around tight spaces.