One of the iPhone's more ridiculous limitations is its inability to sync with host computers and transfer files back and forth. An enterprising developer, however, has written an application that allows not only computer-to-iPhone file transfers via Bluetooth, but also iPhone-to-other mobile device file transfers.
Unfortunately, the Bluetooth file transfer application will only be available through the cydia distribution framework for jailbroken iPhones.
A video, embedded below, of the transfer tool shows files being moved from a Mac to the iPhone, and subsequently from the iPhone to a Sony Ericsson phone:
The tool appears to be based on the work of the BlueSn0w project, which theoretically allows developers to write/implement fully functional Bluetooth stacks that run on your iPhone, thus opening the door for not only file transfers, but also use of Bluetooth keyboards/mice, syncing with host computers and other mobile devices, and more robust support for pairing with third-party Bluetooth devices.