Usually the windshield mount is separate from the unit itself.  My Garmin
C320 is that way and I just pop it out and stick it in the center console.
I guess a thief may still break into your car upon seeing the empty mount
but I sorta think they would likely move on.

I know very little about this, but would love to buy a portable
navigation system. My only concern is that it's very easy to put in
(suction to the windshield or dashboard), but just as easy to steal. I
would think that putting a 500-600 dollar devices in a car that's so
easy to remove is an invitation for the potential thief to break into
the car and take it.

Is there a solution to this problem? Is there a way to install the
portable GPS unit securely in the car? I would hate to take it off any
time I leave my car parked for several hours in the city.

Thanks!