No personal experience as such with the IPAQ, but I do have a GPS antenna for my speed trap detector. The manual on this says that its performance will be affected behind a 'heat reflective', 'comfort' or 'metal oxide' windscreen. Fortunately, I don't have the heated screen.
However, all is not lost. If you get the Tomtom software for the IPAQ I belive this comes with a magnetic mount GPS antenna, like I have for my detector. Strangely enough I find this works in the rear side window. It's shown on my web site at
http://www.mibaillie.co.uk/snoopers6-rneo.html if you want to see what it looks like there.
ian