You can get access to some of the API: http://blackberryclub.com/viewtopic.php?t=270
Unfortunately, no way to connect to a GPS yet at the moment. I suspect you will need to wait until around BlackberryOS 4.1 and the 7100/7290 or thereabouts before it becomes feasible for third parties to develop for Bluetooth peripherals such as Bluetooth GPS. The connector is a client USB only, not a host USB, so you cannot connect any existing GPS -- even RIM couldn't design a GPS driver for the 7280 if they wanted to!
The first modern Blackberry handhelds that would theoretically work with a wide variety of accessories are the 7100 and 7290 with Bluetooth peripherals. Estimated summer 2005 would be my wild guess, if RIM decides to allow Bluetooth API's for RIM-signed applications -- assuming the support of a bare-bones Bluetooth serial profile which would be just about enough to work with a Bluetooth GPS unit.
First version of BlackberryOS with Bluetooth will probably ONLY support headsets, but expect significant pressure by people (including me, consumer Charm users, etc) for RIM to support Bluetooth peripherals in a revision of BlackberryOS.
Mid 2005 is my wild guess before other Bluetooth peripherals begin to become supported. Then we'll perhaps have Bluetooth keyboard support, Bluetooth GPS support, Bluetooth modem support, etc. Hopefully MUCH SOONER, but with typical RIM cautiousness and security concerns, I estimate middle of 2005.
RIM is always cautious about security. It is noted the vast majority of Bluetooth security concerns are mainly about addressbooks (bluejacking), not peripherals. The security considerations of a Bluetooth headphone isn't different from the security considerations of a Bluetooth peripheral like GPS or keyboards, the encryption is essentially the same.