I gather that there is software to deliver mp3s to the RioRecevier that runs on Linux. Will it work on OpenBSD? (My empeg experience with non-Linux Unices is pretty bad.) And is it Java? I don't really want to run Java on my file server.

There are two third party servers, one simple one writen in Perl and one complex one in Java.

Perl: http://www.mock.com/receiver/server/
Java: http://jreceiver.sourceforge.net/

The Perl code should work fine on OpenBSD and as should the Java one (after all people have made jreceiver work on Win2k).

Does the Rio Receiver produce additional gappage between tracks? That is, if my music is encoded so that it doesn't gap on, say, my empeg, will it gap on the Receiver?

I have not noticed it adding any more gaps than the empeg, but to be honest I haven't been looking for them. I bet Tony knows the answer to this though...

The AudioTron seems to have a 30,000 song limit (which is pretty silly). Does the Receiver have any such limitation? Or any other bizarre unexpectable limit?

The Receiver's current code does have an upper limit, I believe it is in the range of 10,000 songs. I think the limitation is in the player code rather than the server.