Thanks for posting this. I have been toying with the idea of having a Mac Mini Server as a mini render farm for a while now.
A few questions.
Are you using the standard 4GB RAM, or did you upgrade it to 8GB or 16GB?
8GB (2x4GB) is very inexpensive now and very easy to fit in a Mac Mini mid-2011.
Have you run CINEBENCH CPU test on it?
CINEBENCH is based on CINEMA 4D, so is very useful for comparing the likely render performance of different configurations. You can get CINEBENCH here:
http://www.maxon.net/downloads/cinebench.html.
The only tests I have seen tested CINEBENCH 11 on a 4GB RAM standard Mac Mini Server model and scored 4.2, you can find these result here:
http://www.barefeats.com/mini11_01.html.
The score for the CINEBENCH CPU MacBook Pro 17 (early 2011), 2.2GHz Quad-core Intel Core i7, 8GB RAM is 5.4, so based on that I would expect the Mac Mini Server with 8GB RAM to score 4.8.
How hot does it run and is it very noisy?
My main worry with these laptop chips, as used in the MacBook Pros, iMacs, and Mac Mini, is that they are designed for short bursts of high performance computing. Rendering on them means running them flat out for hour after hour, maybe for several days at a time, which may increase the chance of system failure. The Xeon, as used in the Mac Pros, seem purpose designed for heavy processing tasks.