Geohot released his blackra1n iphone unlock yesterday. It's got a tweak for carrier unlock on baseband firmware 05.11.07 called blacksn0w that is installable directly from Cydia if you've already got the phone unlocked.
More details here:
Not tried it with another SIM card but I do have the 02-UK text back in the title bar so something has changed.
Update: I had already jailbroken my phone via the pwnage tool. Therefore I only needed to run blacksn0w via Cydia.