HTC has promised (in their usual non-committal way) to release the accelerometer API for the HTC Touch Diamond, so as to allow 3rd party software vendors to implement accelerometer based functionality (e.g. an application that measures how much calories you use when you walk).
Unfortunately HTC has been a bit tardy with this, but that has not stopped enterprising hackers from figuring out the hidden API. Scott from scottandmichelle.net have revealed most of the hidden API, and has even released source code which other developers can use to make their apps also movement enabled.
Read more about it at scottandmichelle.net.