There are alternatives to using a fan. For me quiet operation was critical because my i-opener sits in the stereo rack in the living room. I used a big "honeycomb" type heatsink on the CPU (such as http://www.heatsink-guide.com/alphapal6035.htm - with the fan). The heatsink extents about 1/2" out of the back of the case. With this configuration, my i-opener ran at about 44 deg C playing mp3s in a 69 deg F room. From my understanding, the i-opener can safely run up to about 70 deg C but I wouldn't want to push it much beyond 60 deg C.
If you take the non-fan approach, you should definitely get a temperature monitor program to monitor the CPU and system temperature. You should also use a utility such as Rain to help cool the CPU.