For OpenACS it doesn't matter much, unless you want to use the RPMs. If you do, then I think our RPMs won't work on SuSE, so you'd be stuck with Red Hat or Mandrake.
I am working with Brent Fulgham, the maintainer of the OpenACS and AOLserver Debian packages, to update them. They are very outdated right now.
If you are willing to install "by hand" then any decent distribution where PostgreSQL runs and AOLserver compiles should be fine. I run production OpenACS installations on Debian just fine.
Now if you were talking Oracle, then I'd probably go for Red Hat, but I'd still try Debian first 😊
