Shift Restore in iTunes, and select the 3.0 firmware. When its done, you will get error 1015...Open QuickPWN (3.0 beta) and follow the instructions and it will take it out of DFU mode, and you are downgraded.
Error 1015 is failure to update the baseband...there is no updated baseband from 3.0 to 3.0.1 so it should just go through like any normal restore. It probably won't let you use your back up from 3.0.1.
ultrasn0w works for both 3.0 & 3.0.1. They both have the same baseband, so there is no need to downgrade. Just download redsn0w to your computer & jailbreak your phone (don't install icy, only cydia). Then go to cydia & ad the source that shutter13 gave u. Then turn off 3G. Install ultrasn0w. Reboot the phone. Put the new SIM card in. That's all! Unlocks have become less complicated over time. Downgrading is pretty much useless now.