The MacMini Server is specifically designed to work in low airflow
environments. I've had mine sitting in my entertainment center for
over a year (it's working as my Media center), with no issues due to
heat at all. I've had more problems from the couple drive enclosures
than the hardware they put in there.
There is a company that's been using the Mini for co-located servers
for the last few years. My understanding is that the hardware failure
rate is extremely low, even compared to what you should expect for
commodity hardware.
http://macminicolo.net/
