Some Macs turn off their USB ports when they're asleep. If your mouse will wake the computer (indicating that the USB bus is still active) then your iPod will probably charge fine. Other computers (iBooks and PowerBooks specifically) don't charge while asleep because they turn off their USB systems.
P.S. The new iPods will still charge through firewire, they just don't update that way. The firewire interface isn't built into the iPod anymore, but firewire connections to the dock connector still provide power to the right pins, so charging still works.