There has been a lot of reported vehicle break-ins with satellite navigations units stolen from the vehicles. This I will say is partly due to the growing popularity of satellite navigation units.
Most smartphones have enough screen display size to do GPS justice. One thing I will say is I have never heard of a car being broken into because of a phone cradle in sight but leave a GPS cradle insight and you could be asking for trouble.
In the United Kingdom, you can get a free contract smartphone like the Nokia 6630/6680 and alternatively if windows are your thing you can get an orange SPV c500/c550/c600 or the t-mobile variants, you can get a navigation software with a Bluetooth GPS receiver for less than ₤ 200 and you are good to go.
A lot of the owners leave the windscreen mounts of these devices attached to the windscreen when they detach the units, this is a giveaway signal that leads to the vehicle being broken into with the vandal looking for the device in the dashboard.
These smartphones are really getting smarter, you would want to look for a series 60, 80, or 90 Symbian phone or any windows powered by a mobile phone. Get yourself a compact Bluetooth GPS unit like the holux gpslim 236, that is currently the smallest unit and you are done.