1st, that's no longer the case in CA. You can now have a windshield suction mount in the lower left or lower right corner of your windshield when driving on road.
[EDIT]: Just reread the above and I realize I sound like I'm lecturing. Sorry it was not my intention. Just wanted to share what I've learned about the recent (2009) changes to that law.
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When driving off road, those laws don't apply & you can use a windshield suction cup mount without any issues.
As for the proclip units, I've used a similar unit (purchased at Best Buy) for my GPS on my Audi's vents. They melted to the point they disfigured & fell off when I used the heater on a ski/boarding trip (granted I had the heater cranked for the rear passengers) so I'd verify their usability w/ your heater vents before going that route.