Online bbses aren't new, but aren't common either. I've built a limited Atari ST TOS emulation, capable enough to run a few select Atari ST BBS applications. Namely, BBS Express. The purpose is to bring back the retro feel, of course, and also to provide a platform for ST BBS gaming. (Check it out at http://www.skeleton.org/bbsindex.html, where you can find out hwo to telnet in, etC).
I'm sure others like this are out there...