Sorry this reply is so late, but perhaps someone will find it useful.
My Mac Mini runs FreeBSD 8.0, it's a core2duo 2Ghz with only 1GB RAM, Intel 945GM integrated graphics, Intel HDA (Sigmatel STAC9221) audio.
In 8.0 everything except the remote (LIRC) works pretty much perfectly. Of course you have to jump through a few small hoops if you want to dual boot OS X -- sysinstall doesn't understand GUID disks and will screw it up but you can recover it relatively easily. The important thing I learned was while in OS X use the command line to invoke diskutil to create blank space with no partition on which to install. If you format it with Disk Utility and then convert the partition to a BSD slice in sysinstall there will be trouble (OS X will mount your BSD slice as whatever partition type you initially assigned, and after doing that a few times it will probably be toast). If you don't want to dual-boot then it's pretty much like installing on any PC.
With 8.1 there is currently an issue with sound with the snd_hda driver on the Mini. Hardly my area of expertise, so in fact it may be something as simple as changing the pinout in sysctl.conf. As soon as I find the workaround I will post it here unless someone has already. Meanwhile, 8.0 is working very well here and I am confident 8.1 will be fine for someone who knows more about using sysctl.conf.