My Etrex Vista HCX is less than a year old, never left in car on a hot day, but I had the same problem with the rubber. I've used the unit with a RAM handlebar mount on my bike, and the rubber started to become unglued, especially on top left and right corners, where there's most non-uniform pressure from the mount. I suspect the mount might have accelerated the process, but the real reason is surely a sub-standard glue which became worse on a warm day.
I've picked up some McNett Seam Grip (couldn't find any Loctite Extreme Repair here in Moscow, Russia) and used it to glue the rubber back on. The job I made isn't 100% neat but the glue seems to be holding nicely. Haven't tried it in action yet. I'll post an update when I do.