yeah, i was going to mention Socket's too, but I wasn't too sure about them.
basically people you want to go with a bluetooth usb/cf/sd/etc adapter that uses the WIDCOMM Bluetooth stack/driver for maximum compatibility. I believe the HP iPaqs all use WIDCOMM too. and i think most if not all software utilizing BT is compatible with WIDCOMM drivers (just a hunch)
one way to verify this is to compare the version number (and sometimes the filesize) of the drivers to other WIDCOMM adapters. for example I used Belkin's drivers as a measuring stick and sure enough those other companies using WIDCOMM drivers was version 1.4.2.10. note this was the USB adapters, i didn't compare CF adapters. i assumed it's all the same