Originally Posted by DexKen

It keeps all 3UK settings, wipes everything else (contacts/apps/etc...) but as long as your contacts are synced with google once you login to your google account at reboot all your contacts are synced back to the phone. The market also keeps a list of your purchased apps so you can redownload them.

The first time you launch market it should also list the free apps you downloaded (well it did for me) I downloaded some of them but when I went back to the markets later the ones I didn't re-download were gone, so I'd advise making a list of the free apps you have before upgrading or back them up with Astro.