Update to my situation: Launched Kies today to find there was an update (to Kies itself). Ran said Kies update, still didn't work upgrading my phone's firmware to JB, just locks up after it says it is downloading the update, just like before.
I read that doing a factory reset on the phone will sometimes fix the Kies update issue (I'd been meaning to do that anyway to see if it fixes various performance issues I've been having). No joy. Kies still locks up in the same place.
This is fricking ridiculous. I guess I'll just have to take it to a Sprint store, and hope they can just do it in the 20 minutes I've seen others claim it to take. I certainly don't want to let them have it for a full 24 hours.