I stumbled across a pretty clever use of Mac Mini's today. There is a pair of mini's being used as redundant BGP4 routers on the London INternet eXchange (LINX), by the Fubra network. They apparently installed Ubuntu Linux and used the Quagga Routing Suite for the BGP communications.
The mini's were perfect as they are very compact and draw very little power. Apparently, London data centers have power shortages *shrugs*