Yeah I did it.
I was on a 3g 3.01 JB.
Just put the phone into dfu and restored using a prepwned 3.1 ipsw. Also was using itunes 9.
It went into a restore loop but I managed to get it back by putting it into dfu again and then taking it out by rebooting.
Worked ok but I lost the carrier logo and carrier name in settings.
That would've been ok but I also lost MMS for some reason.
Only way to get it back was to restore to official 3.1, then downgraded to 3.0 and re-jailbroke.
Now I'm on 3.0 JB but with an unlockable baseband. Oh and I needed to run irecovery to get out of recovery mode loop.