I started playing with this. A few pics of my thrown-together prototype:

I went with a simple tractor-trailer arrangement. The tractor portion uses a worm-gear drive in keeping with my ultimate goal of being able to navigate up and down vertical sections of pipe. The spring-loaded top section expands to slightly more than four inches; when compressed inside the pipe, this gives me some traction. Not quite enough, however - I couldn't climb a vertical section, but I could hold my position.

The trailer is attached with a universal joint; this gave me the maximum flexibility for the least work. I'm a little dubious about its strength, however, and will probably use a more elaborate mechanism in the future. The height of the trailer is slightly less than four inches; there's not enough play for it to flop around, but there's no extra tension on the tractor.

I have the RCX attached for weight purposes but powered the unit with a simple battery box.

What did I learn from this? Several things:

• The sections of pipe and the elbows don't fit flush; the inside of the overlap section is actually beveled slightly, meaning that even if you could push the joint tightly together (I couldn't - but a little strategic material removal with a sander would alleviate this), there would still be a slight depression. This causes small wheels like those I used on my trailer to get caught. Either larger tires, tracks, or skis would work better.
• I really wanted more than a single drive axle; I was hoping to have both top and bottom axles on the tractor powered. Doing this in the available space and keeping it light would be a real challenge.
• The actual competition will almost certain require some sort of steering or sensors; as my traction is marginal for a vertical section already, I'm not certain how to handle added weight.
• I'm still not certain how tight of a corner I'll be able to navigate.