For folks who are not in the USA we tendnot to use NAD co-ordinates on our GPS - is there much conversion/tweaking of your conversion routines to allow for this in the route files [e.g. for those of us who use WGS84]?

If you can tell the difference between NAD83 and WGS84, more power to you. The typical GPS receiver you're going to use isn't going to give you that kind of accuracy, unless you drop a lot on one. The "proj.4" library includes everything needed to convert, but you again, you're probably going to not care.

For what it's worth, I use Xastir with NAD83 calibrated data and a GPS receiver set to WGS84. The errors are always either "compromises have to be made when drawing a map" or "GPS receiver can't see enough satellites"

Using aerial photos when doing GPS tracking is way cool, but they're big, data-wise.