For those looking to use a microphone with the mac mini...
There are two 3.5mm jack sockets on the back of the mac mini:
1. Line-out - three connections - stereo left out, stereo right out and microphone in.
2. Line-in - two connections - stereo left in and stereo right in.
Apparently (I didn't try it) you can use the ear bud/microphone combo supplied with an iPhone in the line-out socket - this is a pain, because it means you have to disconnect the powered speakers you would usually use in this socket.
The other option is to use line-in, but this only accepts a condensor microphone signal, which also requires a pre-ampifier.
A third option is to use an iRig PRE (see google for details) which functions as a pre-amp for condensor microphones for iPhone/iPad. This can be connected to the mac mini line-out socket and the "monitor/headphones" socket on the iRig PRE can be used to connect powered speakers - everything ends up connected to the line-out socket, but it all works.
One thing to beware of if using pulseaudio with the above (on my hardware it shows as "internal microphone") - pulseaudio assumes that the microphone supplies a stereo signal and locks the channels together - with a mono microphone, this has the effect of cancelling out the signal. In order for things to work, the channels need to be unlocked by clicking on the appropriate icon.
The result (as commented by the party at the other end) is very good quality audio with skype.
Edit: In case anybody is wondering, there's some kind of issue with the kernel that prevents a usb headset from working (apparently ongoing for a couple of years) and some other (ssp mode) issue that prevents a bluetooth headset from working (it used to work)