I found this on Voipinfo.org and might be handy for people like me who travel frequently. Many a times I find myself with blinking "NO SERVICE" when I travel outside of USA. I do have SIM cards from other countries that I have paid for but they do not work with my iPhone. So it is always my blackberry that works, everywhere.
According to the post and the information on the website, the unlocking SIM card for iPhone seem to work with OS 2.0 and the latest iPhone OS 3.0 Betas and supposed to work with different carriers worldwide, such as AT&T, T-Mobile, O2 and others.
The following information was taken from the Mr. Unlock.net but as I am not sure of the legality of this product I will provide you a link to the voipinfo.org. You will be able to find your way from there.
● FULL Unlock for Any Network! Worldwide!
● ALL Functions Working!(Incl. 3G Data network and EDGE!)
● Very Easy unlock, no knowledge required!
● No iTunes Sync!
● This is the ONLY Reliable & Stable unlock Solution for the iPhone 3G!
● No Signal Loss like other Unlocks available!
● Instant Access to 1000's of iPhone Applications!
● 100% Working, 100% Reliable, 100% Safe!
● Full Money Back Guaranteed!