As kimsland mentioned, it will vary from hardware to hardware. Also, maximum and minimum temperatures for all hardware will change based whether or not the equipment is in use. They'll also usually specify a humidity rating as well.
You, however, are referring specifically to the operating temperature and the warnign / shutdown temperatures. So you know, many motherboards actually let you configure this yourself - on my Asus M2N SLI Deluxe, for instance, you can configure a shutdown temperature for the CPU, the chassis and the chipset, as well as warning temperatures for them as well.
Also note, these figures are separate from the CPU's programmed shutdown temperature as well, which is something that all modern desktop CPUs have. This is much higher than the board shutdown temperature - usually north of 90C, I do not remember specifics.
Typically, a motherboards default shutdown settings will be in the 70-80C range. The default is usually just fine, and if you are experiencing shutdowns due to high temperatures you should probably take a look at why it is overheating before you increase the temperature threshold.