Yes, I do have also an iPhone and yes, I want to give a chance to other good products also, not just for Nokia Maemo devices:) If you accidentally upgraded iPhone’s firmware to 3.1.2 which also updated the baseband, you were out of luck. The firmware was not possible to unlock or to downgrade to the previous version. Not before today.
A hacker called geohot released a new unlock for iPhone called blacksn0w today. It was first available only with blackra1n, but now it’s also available via Cydia if your iPhone’s been jailbreaked already. I guess you can find these instruction like billion times from the internet but here’s one quick guide more.