Damn right!

If they SEE IT,they will try to steal it.
"Securely mounted" or not.
They will RIP APART your car to take it.

Maybe you could disguise it,have a fake kleenex holder that drops over
it,so it does not look like anything that is worth stealing.

Or mount it IN the dash,and have a panel that closes over it when not in
use(or so it IS the flip-up panel).Some people remove the passenger airbag
and install their DVD screen there,it flips up when in use.
You -could- do the same for your nav screen.(I don't recommend it.)

The trick is that they not see it.

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!