I don't use any fancy pad, I use standard pipe insulation, mainly for protection against the mtn bikes and beaters that are sometimes racked up next to(read: on top of) my bike. kinda like this:
http://doityourself.com/store/0679738.htm
Wrap it with a bit of electrical tape and you're ready to go. Protects the whole top tube while my bike is on the train. When I have my good bike I carry a couple of little pads for the seatstays as well.