Now that I have been there, I would have to say the 17-55 would probably be just about the best one lens solution at Disney. I was extremely happy with the 15-85, and the 15mm length was used way more than I expected, but the extra speed of the 17-55 would have been welcome some times.
I probably would have been pretty happy walking around with my Sigma 30mm as well, except right before going I noticed some focus issues with it, so I simply didn't trust it as a single lens solution.
Changing lenses at Disney is a HUGE pain IMO. I'm still happy I took my DSLR over a P&S, and with a decent small backpack you hardly notice the weight. Having a good all around one lens solution is the ticket as far as I'm concerned. I think any of the 17-50(VC), 17-55, 17-85, 15-85 lenses is a good choice. Having some type of image stabilization is almost a must IMO if you want to get some cool night shots (Epcot, the castle, etc.) without trying to lug a tripod through dense crowds of thousands of people.