More batteries....
One thing to consider with Bluetooth GPS receivers is that they are battery powered and as such you'll need to carry spares for those moments when it fails (at the worse possible time).
I find a CF GPS receiver best as it only sticks out by 3-4 cms out of the top and gets it power from the iPaq. Got mine for about £50 on ebay and use it with an iPaq 2215.
It may be an old fashioned solution but it works for me in the car and in the air.
esvdx