The possibility of this is pretty low. The Mac minis didn't have much in the way of expansion anyway. You might be able to get faster (but not Gigabit) speed by leveraging the Firewire port and another Mac *with* Gigabit.Someone is bound to turn up with a solution now that you've raised this!![]()
Best hope would likely be if an Ethernet card was made for the slot reserved for the AirPort Extreme card. Of course, that will involve reverse-engineering that slot, if it hasn't been done already.