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/

For all those who have upgraded to the Iphone firmware v 3.1.2 an Unlock was released yesterday by GeoHot.
Blackra!n RC3, the Jailbreak tool is available at http://blackra1n.com and includes the new Unlock. There are versions available for both PC and MAC.

For those who have already jailbroken their Iphones running v 3.1.2, Blacksn0w can be downloaded via Cydia by adding the source http://blackra1n.com to Cydia.
Enjoy!