The folding luggage carts described here are still available at Staples, but they are surprisingly more expensive than you'd expect. I saw a 100lb capacity model at $32.99. I seem to remember paying about $19 just a few years ago.

Also, I agree 100% with the statement above about lugging around an extra piece of gear to provide a minor convenience. Same problem with a wheeled pannier. You'd need a long handle, which would add extra weight and consume a good amount of space within the bag.