I believe some of them use Linux but a quick look on the 'net hasn't revealed exactly what the others use. Given this device isn't actually a TomTom I can't categorically state these other [TomTom's] are WinCE but I thought it might imply that since their software worked on this WinCE 4.2 GPS....
Anyway I'm interested to find out myself now so if I come across anything I'll pass on the info - I see you're reasonably active on the Pockfms forums so I guess it could be of some use.
Cheers, FP.