Preliminary reports say that the OS is downgrading but the modem firmware is not (aka, the modem firmware stays at 04.01.13_G instead of downgrading to 03.14.08_G). This allows use of Wi-Fi but not GSM functions (phone/edge/sms) unless you're using AT&T.
It appears to provide a method to bring the iPhone back to the 1.0.2, however the modem's firmware remains unchanged, and does not appear to work (no phone functionality), so is not a practical general solution at this time. But, if you do have a SIM Unlocked iPhone and prematurely upgraded to 1.1.1 already, this could provide a method to restore functionality of your iPhone.
SIM Unlocked iPhone users who have not upgraded should still avoid upgrading to 1.1.1 as you will lose your SIM unlock when the modem firmware gets upgraded. At this time the modem firmware upgrade is irreversible.