I think the PocketFMS team is actually working on porting their software to road navigation devices, including the TomTom. Don't know the status though, and don't know for sure if the TomTom is included and if so, what models.
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Edited: Checked the PocketFMS website, couldn't find anything wrt. the standalone TomTom devices. Lots of info on operating the TomTom software in conjunction with PocketFMS on the same PocketPC hardware but that's not what you're looking for.
PocketFMS will work on many many car sat nav units, but they must be based on the WinCE operating system.
Unfortunately both TomTom and Garmin Nuvi car sat nav's do not use with WinCE operating system, and can't be used with PocketFMS.
If you haven't yet purchased your car sat nav, then it's worth deciding before you buy it, if you want to use PocketFMS (or other software) with it first, as it will obviously affect your decision.
When looking for such a device, it's also good to check out the screen when outside in sunlight. Some of them are washed out and hard to see, and some of the better ones (eg. Navigon 8110, Mio C320) are excellent and easy to read in daylight. It makes a world of difference in the cockpit.
dp