Here’s the best 3.1 Jailbreak guide out there http://www.iclarified.com/entry/index.php?enid=4254&laid=29.

So, after updating to 3.0.1 on Saturday and using redsn0w to jailbreak with 3.0 IPSW I updated my phone to 3.0.1 but had issues with PUSH loosing my Push Fix that was working and also getting the YouTube “Cannot connect to Itunes”. So I decided today to restore again but after reading several times that using a USA AT&T SIM fix the Push issue with out needing the Push Fix pkg from cydia I thought I would give it a try. The result: IT WORKED EVEN being overseas.

1. I updated to 3.0.1 using Itunes, be aware if your device is JailBroken it will loose it so backup your cydia apps google how to do it if you don’t know.
2. put the AT&T SIM and use redsn0w to jailbreak using the 3.0 IPSW don’t worry you’ll keep the 3.0.1 Firmware.
3. After jailbroken open up Itunes again and when asked select SETUP A NEW PHONE (you’ll re-sync all your data contacts, music, apps so don’t worry about not restoring the data from the last backup). After this you’ll get a pop-up that says waiting for activation this may take sometime while this happens all your data should be syncing…
4. Open up Cydia you’ll add ultrasn0w’s repo more info here it will ask you to reboot before doing so put your local SIM in (remove AT&T’s) and reboot.
5. Your Done! on reboot your iPhone Should be activated and working perfectly including Push notifications and YouTube enjoy!

Credits:
DEV-TEAM developers of ultrasn0w FREE SOFTWARE!
@MuscleNerd (via twitter)
@PlanetBeing (via twitter)
@iphone_dev (via twitter)

Sorry not available in English yet :p.