I was looking for a Bluetooth adaptor which would project as little as possible from my laptop so as to make it more robust to leave plugged in while carrying the laptop in a bag.  

An option might be a CardBus or PCMCIA card. There seems to be at least one which projects only a small amount (6mm?): Zoom 4312AF, and one which might project not at all: Socket BL4539-642 (CF card w. PCMCIA adapter), either seems difficult to buy in NZ. However my laptop's CardBus slot is occupied by a 3CRPAG175B X-JACK wi-fi card. 

This might also be of interest to owners of ASUS Eee PCs who do not wish to make internal hardware modifications to provide BT. 

So here are a couple of USB BT adaptors which project only a small distance out from a USB socket.
BTEU-035 www.jaycar.co.nz
Princeton PTM-UBT3S www.conics.net
both have images of the devices. 

The BTEU in plan-view has a curved end a little over a half-circle. It projects about 11.5mm when plugged into my laptop. The Princeton has a rectangular end and projects about 9.0mm. The Princeton originally had a couple of very small horns as vertical edges on the outer corners probably intended to make it easier to grip and extract from the USB socket however I filed those off. 

Both are supplied with Toshiba BT software, but as I need only connection to ActiveSync on my mobile 'phone, GPS connection, BT modem (mobile 'phone) and as I am familiar with setting up the MS stack. I opted not to install the Toshiba stack. Both devices were recognised by Windows XP Prof (SP2). Installing the Toshiba stack apparently removes the MS stack - I would be interested to hear whether others think there are any negative connotations re installing the Toshiba stack.
 
 Although the Princeton is even smaller than the BTEU, I favour the BTEU as I think the curved end makes it less likely to catch in the bag.