Older GSM phones could be hooked up to a PC and you could access the baseband chip through e.g. minicom or hyperterminal and give it AT commands, usually through a serial/rs232 or USB connection between the phone and the PC.
A lot of software also exists to use GSM phones as e.g. SMS gateway - which basically works by getting a serial connection to the phone and communicating with AT commands. Can one do the same with a HTC Desire, or other Android phones ?