GP, one of the largest manufacturers of rechargeable batteries defines
0-60 degrees as working temperature
0-55 degrees is the range the sensor is designed for and accurate at.
90-98 degrees is the auto shutdown temperature.

It's safe unless you exceed 98 degrees.

Typically in electronic systems, if you supply enough voltage, and keep the temperature low enough, transistors will work.

I had my N900 work in a 44 degrees (outside) all of yesterday, it works fine, I'm sure the internal temperature was higher then this.

When it's more then 50 degrees, it's not comfortable in your pocket anymore.