I have the same question as OP I have an iPhone 4 on BB 01.59.00 and installed ios 4.3.1 via PwnageTool. I went to Cydia and installed ultrasn0w 1.2.1 on my phone.
So besides buying or using another sim card is there a way to tell if it is unlocked for sure? Or is it pretty much for sure unlocked once I used ultrasn0w?
Thanks for your help