I just jailbroke my phone that I got yesterday. Now the modem firmware is 05.12.07 is there anyway I can downgrade to 05.11.07 so that I can unlock it or am I screwed?
Actually, if you have a 3G you might have a chance of having bootloader 5.08 which can downgrade the baseband. I have one and I used Fuzzyband Downgrader from Cydia to downgrade my baseband to unlock my phone.
Not all 3G's have this (actually not a lot of them at all but I wasn't expect myself to have one either!) but try downloading Fuzzyband Downgrader from Cydia and trying it. It will tell you what bootloader you have. I mean don't get all excited cuz you might have bootloader 5.09 but I'm just saying...try it before you sell it.
The 3G Fuzzyband Downgrader from BigBoss on Cydia does not support 3.x firmware. Look for the app named just Fuzzyband from BigBoss, that is the latest version.