The Marathon Open Source page has put up another update, this time on the progress of Carbonization and the addition of networking:

Loren has also made some good progress toward bringing networking into the Classic version, but we've hit some nontrivial roadblocks along the way. In particular, getting the packet-listener to run when a new packet arrives is a problem. The SDL versions have a high-priority thread that's blocked on a select() or read() call (or something like that) most of the time. Bungie's code used an AppleTalk mechanism to have the OS call into your own code.

The full article has more technical details on what exactly the problem is, as well as the proposed solution.