There have been reports of WiFi and GPS issues with Iphones running 3.1.2 unlocked with Blacksn0w.
A fix has been posted which entails resetting the network settings and installing a Push application from Cydia.
Read about it at:
http://www.redmondpie.com/fix-blacksn0w-wifi-youtube-gps-problem-on-iphone-3.1.2-firmware-9140085/
