AT&T finally announced the Skyrocket Jelly Bean upgrade (with Butter) on April 10, 2013. Samsung posted detailed instructions.
Similar to the Skyrocket upgrade from Gingerbread to Ice Cream Sandwich, the Jelly Bean upgrade was only available using Samsung's Kies.
As called for, I upgraded Kies. As I read and re-read the upgrade instructions the Jelly Bean update downloaded on its own. As this was the same laptop I used for Gingerbread to Ice Cream Sandwich all the necessary drivers loaded cleanly.
Unlike the Gingerbread to Ice Cream Sandwich upgrade I didn't remove the SIM card nor the microSD card. The FAQ specifically said that you could leave them in. I had no problems with either. I did not lose my internal SD storage either. I did not do a factory reset before nor after the upgrade although that is usually a good idea.
The upgrade went perfectly and took about 25 minutes. The laptop is an old IBM ThinkPad T43 and I could see the processor busy most of the time. I'd expect it to be quicker on a more powerful PC.
After the upgrade the first boot went through the normal "optimizing" phase and sometimes I'd see an app start slowly the first time. I presume that this is the Just In Time (JIT) compilation happening.
The Jelly Bean for the Skyrocket is 4.1.2 but I haven't noticed any of the differences between this and the Captivate's 4.2.1. As expected all of the Samsung customizations are carried forward, e.g. TouchWiz and the camera. And you lose a Jelly Bean feature here and there like the widgets on the lock screen. Maybe I just haven't figured that out.
You might want to screenshot your home screens before you start as the upgrade resets them. It also forgets ringtones, etc. Most apps kept their settings.
Jelly Bean is so nice. Here's a screen shot of the in call screen. iPhone users, eat your heart out.
And Google Now, just wow!
I had Googled "Happy Daze" on my laptop looking for the hours of a local hamburger joint. Look at what Google Now did with that signal.
There's a lot of whining in the Android forums about the carriers not upgrading their older phones. While all of us geeks want the latest bits as soon as they are available, AT&T and Samsung have upgraded both the Captivate and Skyrocket twice in less than 2 years. I really can't complain about that.