Good news. The later TomTom units have the SirfStar III chipset, which isn't affected by the athermic screen. I'm not sure of other manufacturers' units, but when it comes to the TomTom line-up, they're split as follows...
Models that have problems with athermic screens:
Classic (the original 'Go' with no model number)
Models that don't have problems:
If you have the small untinted square at the bottom centre of the screen, then you do have the athermic one. Our 04 Clio has that, whereas the 02 didn't, and our problems with using my 700 in the Clio only started when we swapped cars.
Somewhat annoyingly, it's impossible to mount a TomTom Go in such a way that it can actually 'see' the satellites through that little window anyway
. Fortunately, Currys were offloading the official antenna kits at a tenner a go a few months back to clear the last of their stock, so I got one of those and positioned the antenna so that it can see through the sunroof.
Heated windscreens don't seem to cause as much of a problem as the athermic ones, though some people have reported problems with them. I tried my 700 in a colleague's Mondeo and it was fine.