folks just went thru this unlock business with ATT and iphone 4.
can't speak about other phone, but process unlocking iphone 4 is a pita! unlock doesn't mean you can use it on other carriers. this is due to lack of capacity to change APN Carrier settings.
sure your iphone is unlocked, but without a means to change your APN settings. your phone is still locked to ATT or who ever.
there's two ways to access your APN settings with iphone.
1. download a APN tool targeted at corporate customers from Apple's site. be prepared to find this yourself as Apple support knows nothing about existence of this tool.
2. visit a website to download an app that changes APN settings for you. visit on iphone in wifi mode, then follow instructions: http://www.unlockit.co.nz/unlockit/