I've left my DS/9 out in the sun for several hours with no apparent ill effects. But it's not quite summer here yet, only 80-85 degF out. We'll see in July when it's in the high 90s. [I'm also curious to hear from some more northern dwellers how it works when it's 20 degF out, though no one may be crazy enough to ride in that weather.]
I presume there's a thermal cutout on the batteries, but I suspect thet the motor's thermal limiter would kick in first.
I usually cover it, though, so it doesn't get spray-painted, misted by cooling towers, etc., as I work at an industrial facility.
This kinda leads into what would be one of my wish-list items for the Zero: a full digital dash that could tap into the bike's CAN or whatever bus it uses and display info like this. I understand their OEMing a generic unit like they did, to reduce cost and development time, but having temperature info, instantaneous kW and cumulative kWh readings, kWh when charging (no more Kill-a-Watt needed), etc. would be uber-cool. Besides, I never look at the speedo needle, just the digital numbers. Seems like such a waste of space