No restore needed, thank goodness. In order for purplesn0w to work, once you download it in Cydia, you need to reset network settings and disable 3G. I'm on Apple approved carrier (ROBBERS, I mean Rogers), but you still needed to disable 3G. Once I did that it worked fine, but Edge was slow as hell (96 Kbits/s down and 48 Kbits up, compared to 4109 Kbits down and 264 Kbits up on 3G).
I think I'll just stick to ultrasn0w if I need to take my 3GS to Florida (I use T-Mobile in FLA), but again I still have my 2G nicely JB'd and unlocked on 3.0. Another nice thing about ultrasn0w is the SBSettings ON/OFF toggle.