Someone just pointed this out to me:

Midi over TCP
http://www.musiclab.com/products/rpl_info.htm

Why cross-platform? MIDIoverLAN CP communicates not only PC computers. Our new solution can now communicate PC and Mac computers into unified MIDI network.


Ever wished to flow MIDI stream over the local network without torturous setups of MIDI interfaces and cables? Want to get a fast MIDI connection between different applications in your computer? We are proud to announce MusicLab MIDIoverLAN CP - cross-platform solution for these problems.