Toby,
I just bought Cabela's "Deluxe Harness and Climbing System" ($55) which I've used a few times already. It is comfortable, has D-loops on each side of the harness for climbing, and the safety webbing doubles as a climbing rope. The safety webbing has D-clips on each end, so you can use it to climb. Once in the stand, you unclip from your harness and secure it around the tree, then use one clip to clip back on to the short piece extending from the back of your harness. The worst part of it is trying to figure out how to put the contrapion on in the dark.