Assuming that your mining machine is a 10cm * 10cm * 10cm box with 100W power, while it is a black body, then according to the Stefan–Boltzmann law, the temperature of your machine will be......

414K, or 285°F.

One of the hot-selling mining machines in China is the Friedcat Box, providing 38GH/s in 360W power. The dimensions of this thing is a 12cm cube. Using the data of this machine, and calculated with Stefan–Boltzmann law, the temperature will be...

520.6K or 477.5°F

Bitcoins are really hot, I think.