The Mooglex Sim Card Adapter has proven to unlock the iPhone 3g and 2g without voiding the warranty of the phone.
More importantly it currently supports the new Apple update software, 2.0.1, for the iPhone 3g. Previous rumors of the iPhone 3g rendered useless after updating with the unlock Sim Card has been debunked.
Thank you mooglex!
You can visit them @ mooglex.com