So you're gonna be hiking around Nevada in June? My first question is ARE YOU NUTS? ;-)

Seriously, I've left my MBP in it's Brenthaven case in the trunk in the summer here in Alabama (with it powered off) and have had no problems. I have, however, allowed it to cool in front of an A/C vent before powering it up when it is REALLY warm. I also bury it under other stuff in the trunk, but that's more for security concerns. (Not that it would prevent someone who breaks into my trunk from uncovering it...)