Andrew Gray mentioned his own work with Rotor and FreeBSD 5.1:
I had a look at building Rotor on FreeBSD 5.1 recently. I resolved some issues, but did not get it working.
The following newsgroup posting (linked above) includes a patch for the things I resolved and some notes on the unresolved issues.
Even though Andrew was not able to get Rotor working on FreeBSD 5.1, these are some great notes about problems he resolved as well as a good starting point to the next phase of research. Thanks Andrew!
I have been setting up FreeBSD 5.1 under Virtual PC tonight, so I am heading that way myself. More to follow ...