I have an iPhone 3GS running iOS 5 and baseband 5.16. After reading various blog posts and threads, I was told that in order to unlock it, I had to first upgrade it to the iPad baseband (6.15) and then downgrade it. Well, when I tried upgrading to 6.15, it took me halfway through the process and then said I had to first activate the iPhone. Not having a SIM card yet, I couldn't proceed. Now, WiFi doesn't work, Bluetooth doesn't work, calls, tons of things. The modem firmware is blank. The top left of the screen just says "Searching", and iTunes won't even recognize it. I think it may be bricked. Any suggestions?
Could it be that your iPhone 3GS was not compatible with the iPad baseband ?
The iPad baseband is not compatible with 3GS units manufactured week 34 of 2011 or later. If you have an iPhone3GS and if digits 3-5 of its Serial Number are 134 or later (xx134…), then you should NOT try to install the 06.15 baseband on your 3GS! It will brick your radio, preventing both the downgrade from working and normal iPhone software from using it as a phone! Be warned!
See this post on the iPhone Dev Team blog.