3Gs Jailbreak for 3.1 released by the Dev-team, but beware: only works on Mac OS X; will only work on previously jailbroken 3.0 or 3.0.1 iPhones. Don’t expect a preinstalled out-of-the-box 3.1 to be jailbreakable. Read carfully all the conditons in order to avoid irreversible damages to your device. Use at your own risk.
My personal opinion: if you have a perfectly running jailbroken iPhone 3.0.1 and are not missing on apps that require 3.1. to run (dropbox will upgrade to be backward compatible), keep it that way and wait for the next update that fixes the random shutdowns and frozen screen issues