I have a site on OpenACS 5.2.3 on Postgres that I am porting to Ruby on Rails. It is a side project, for fun.
I'm preserving the OpenACS data model, although I don't necessarily plan to keep the same data model once it is up and running.
The packages I plan to port are:
* forums
* blogger
* news
* resource list
and I'll probably end up doing more as well.
The project lives here on GitHub:
http://github.com/jadeforrest/OpenACS-on-Rails
Note that I'm not doing a general port -- I'm just doing the parts that I need. But with git, you can largely handle the problem of forking your own codebase and keeping it unique.