This sauna is not a steam sauna -- dry heat. So not worried about the moisture but good point. I was just wondering what the safe temperature would be -- if its in any official document.
Taking something from hot - to -cold - to - hot will cause it to "fog up".
Dry sauna or wet sauna, hot is hot. I doubt you live in a zero humidity area, so your 'Berry will fog up.
My Blackberry Curve suffered some water damage so everything works except all the buttons. In the past I had a phone that fell into the toilet, lol, and stopped working completely and after a couple of days and being in the oven at 150 F it started working fine. This blackberry i put in the oven for a couple hours at 145 and even though it didnt fix it, it also didnt mess it up more than it already was. So at least 145 is safe, but I would call RIM and ask them. That would be your best bet.