I had a stock xperia 2.1 install on my pro 10 U20i, i did the jail break on my mac using break droid which apparently does all androids, one click jailbreak – downloads here
once i had it jailbroken i did a search on market for root check which confirms the break. Then i installed busy box from the market again.
i need to get the clockwork restore manager on the phone now which I have seen can be done by x-parts from market which I found later I used a console on ubuntu, also a windows version available here
now the we have root, busybox and cwninstaller we can upgrade to 2.3.7 using cyanocream! which I followed these instructions and it worked great
I can install more apps aswell, apps such as shazam and instagram woouldnt install on 2.1 but will on gingerbread.
plus the new mod comes complete with google play
handy wiki for problems after flashing your rom here
maybe ill go for the ICS upgrade soon which looks real busy!
I did manage to brick the phone when i first tried recoveryx which made me panic, i just loaded up for sony bridge for mac and selected repair which took the phone back to stock install -> phew!!!