I have successfully used pocket pcs in the past (using the serial port) to control simple robots (small rovers).
Looking around here and on apples' developer website, it seems that starting on 3.0, they do allow applications to communicate (and even use their own protocol) with custom hardware. I'd assume they have the same policy for bluetooth communication.
But it seems the hardware developer program is geared towards big corporations
Is it possible to make non-jailbroken iphones to communicate with custom hardware (a PIC microcontroller with some bluetooth chip interface for example)?