I don't have OS2008 installed yet, but my BT TomTom GPS Mark I works fine with Maemo Mapper on OS2007. But:
1) There's no need to pair with it (as in pairing in the control panel), and
2) If Maemo Mapper has been set up to work with one BT GPS (it takes note of its BT MAC address) it won't work with another BT GPS (the N800 sees it but MM won't use it). You have to go through MM's BT GPS configuration again. I was bitten by this one when I couldn't get MM to work with my friend's GPS (exactly the same as mine).