I really appreciate all of the replies to this thread. They've really helped me keep my spirits up about this over the last few days.
We went to the vet on Monday and they unwrapped her leg and noticed she didn't respond to any pressure on it, so it definitely had to be removed. I left her there overnight and they performed the surgery to remove the leg yesterday. She's apparently recovering great because they let me bring her home tonight!
The biggest thing now is to keep her calm for the next week or so. She's already wanting to go running around. Argh. How do you explain "downtime" to an energetic cat?

I've got some pictures here, if anyone is interested in what a tripod cat looks like a day after the surgery. They're not gross-gross, but it was shocking to me to see her stitches and stuff for the first time, so if you're queasy about that kind of stuff, wait a few weeks and I'll post "her being normal again" pictures.
http://gallery.mac.com/jason.thorn#100029&bgcolor=black