Ah, grex! I used them when I lived in Ann Abor. I remember when grex was just grex and they had a vote on a domain name (cyberspace.org).
So if they've been running OpenBSD for years, why the switch?
Arbornet is another one that runs FreeBSD, and is also in Ann Arbor:
http://www.arbornet.org/free_shell_accounts.php
Running a free shell provider sounds like fun. You can rent a halfways decent dedicated server with software and plenty of bandwidth for $50-60 a month so it's not a huge investment dollar-wise, though I imagine weeding out abuse would be a headache. Then again, your typical abusive user is looking for cpanel on linux so they can upload their spammy scripts, not a BSD shell.
Anyone who runs one care to comment on the challenges?