The Dev Team did it again in style by releasing Pwnage tool for 3.1.3 iPhone Firmware. 3.1.3 Firmware from Apple is not a noteworthy upgrade and it is suggested you skip it! Few bug fixes, battery fixes, unlock-mess ups are everything Apple has to offer with 3.1.3 firmware! Some of us just like the higher software version and there is nothing stopping us from trying it, eh?
3GS users should proceed cautiously. No kidding! Drop a comment if you have any questions – Read this real quick and only then should you proceed!
I would suggest you read the official announcement by the Dev Team.
Stay tuned for Custom Firmware for Windows Users and some Mac users (the rest can create them on their Macs, right?!). I will personally test the custom firmware on my 2G, 3G and 3GS iPhone and only then release them today!