Inferno (and Limbo) has its own mailing list. Talk on those topics isn't particularly discouraged here, as there's obviously lots of overlap in both ideas and community, but it mostly lives on elsewhere.
They're conceptually very similar, and share much of their implementation. There are important differences, however. Limbo only really runs within Inferno (which can then run on top of Plan 9, FreeBSD, Windows, whatever), and you can't run user-mode C code in Inferno. If you're looking for rio and fossil in particular, you need to use Plan 9 (or 9vx). There is an acme for Inferno. If you want something to fill a semi-traditional server or workstation role, you probably want Plan 9. If you want a neat application programming environment, or an environment for embedded systems, you probably want Inferno. But neither are always true. Most of the time i wish the two were more similar. Or maybe more interchangable.
