Celeste
I just upgraded to 4.2.1 primarily to purchase Celeste...
After purchasing, I didn't get an app screen, I can't find an option to toggle it on/off in SBSettings anywhere, and when I go to my songs there's no option anywhere to send via Bluetooth.
I'm not sure if it's actually installed, but Cydia says it is. It could just be filled with bugs and not grooved out yet. It would be informative to give some type of instructions on how to use it after downloading, there's little info on the net and I'm completely stuck even just figuring out if it's installed and integrated in to my app's yet or not.
My advice would just be to wait to upgrade until there's a good release out for it. I wouldn't waste your time right now, I was looking forward to its features but haven't found a hint of them anywhere yet.