Do you mean an integrated unit or something you can remove?
I have a TomTom Start25 that, with a little bit of surgery to the cubby floor fits. You can see it on my build thread here ...
http://www.350z-uk.c...re-strut-brace/
The cubby lid is still fully functional and this particular TomTom model has a "start up when power on" feature (not all of them do) so when I turn on the ignition, the unit powers up automatically and powers down when I turn the ingition off.
David