If they're like me, they have a Mini with an internal 4200 rpm drive. So if you decide to boot to a new fast external HDD for more speed, and let the internal 4200 rpm drive do the Time Machine, you're in good shape.
At least, that's the hope. I'll know soon. I have an external 7200 rpm external drive ready to be my new boot drive when my copy of Leopard is in hand.