Well, you know, that will depend on several things, like ambient room temp, and the processor load etc.
That said, the mini fan vents from the rear, so make sure that space is open. Also, try and have some clearance below the mini, as it draws some air from there (and definitely don't stack the mini on a receiver or other "hot" device, as the mini's own heatsink is on the bottom).
Anyway, it'll probably be fine, just get something like iStat to monitor temperature and load. If it's getting too hot regularly, get something like SMCfan control and up the default fan speed to keep it from ramping up on its own (i.e. set the fan speed to cool the computer constantly).
Good luck, I'm sure others will have some real-world experience with this type of application for your mini.