Thanks to everyone who responded to my question about IP MIDI solutions. My understanding is that the current options are:

EthernetMIDI (freeware, PC only right now)
NetMIDI (commercial, dual-platform, only works with proprietary Giga PC's assembled by that particular company)
MidiOverLAN (commercial, dual-platform)

So it seems that if you want to send MIDI to your GigaPC from a Mac, and you don't want to buy a custom PC, MidiOverLAN is the only option.

Does this seem right? Are there any other possibilities I've overlooked?