You well have to check the bootloader version, just google it, or install Fuzzyband from Cydia. If your iphone's bootloader is 5.9, it's impossible to use yellowsn0w because of the upgraded baseband, but if its 5.8 or any other version, it might be possible.
I have the 5.9 version, so im stuck now! waiting for the next update which is coming by the end of today.