As i mentioned in another thread, my 7d has been used in hot 100+ degree days and used in snow storms and rain and has been a champ... BUT... as of recently, I have a feeling my cameras weathersealing is drying up/wearing because on a few recent paid shoots outdoors in freezing temps, it is starting to freeze up on me (top LCD looks like it's working but will no focus, fire, shut off, basically non responsive... Taking the batteries out and reinstalling them is the only thing that gets it working again until it decides it's too cold to operate again... then rinse repeat. Keep in mind i was one of the first to buy my camera in my area when it first came out and this is the first time it has ever malfunctioned on me. It has/had wonderful weathersealing but they can dry out over time. I would never trust a camera in a car unsupervised unless it's in the winter, but even at that, I'm skittish because it's not a cheap item to be leaving in a car. Do you have a locker at your work? An office? perhaps get a small bag you can fit a body and lens or two (not a big backpack) that may be more work friendly? Then again for me, i'm paranoid enough where even if I have to go to a store (grocery store/walmart/etc) my camera is coming with me... I may get funny looks but I know my gear is where it should be, with me.
Lastly, regarding a rebel body, things to consider about having in in a hot car... think about glass heating up in the lenses to the point they dont focus properly... cracks in the plastic of the body, rear LCD screen, mirror, focus screen, etc... so many things could go wrong so quickly... For me it's just too much to risk damaging.