you can't jailbreak 3.1 Firmware as of yet
I did it when I first got my iPhone and really didn't care too much for it....I mean besides adding ringtones and custom icons it did nothing for me...
plus in order to tweak the iphone after being jailbroken you have to use a SSH program and to me its way too much work....
for Ringtones I bought a program called RingTone Expression....allows u to create ringtones and upload them to your iPhone in seconds.