The iPhone Dev Team's Pwnage tool is set for public release tomorrow. The tool allows you to upgrade any iPhone with the latest firmware from Apple, modified to make it work with any compatible carrier in any part of the world, and install any application, as well as adding custom payloads, additional software like SSH, FTP, and other applications.
Update: Due to legalities the Pwnage tool has been delayed. We'll keep you updated with any release information as we get it. [iPhone Dev Team]