The Inferno kernel was created by modifying a then-current Plan 9 kernel. I believe that they do share device drivers, but the Vitanuova folks can probably answer that better than I can.
See the Plan 9 and Inferno manuals and nemo's Plan 9 3rd edition kernel commentary for design and implementation papers. I believe that the current Styx will interoperate with 9P2000, but haven't tried it myself. Limbo is an attractive language in its own right, more so for concurrent applications than the lower-level uses of C. Its facilities for writing concurrent programs are far more pleasant than something like Posix pthreads. Portability doesn't strike me as a basis for choosing Limbo (on Inferno) over C on Plan 9. Both rely on the portability of the underlying operating system. Given that you're unlikely to have the right hardware to usefully run native Inferno, you'll have to run Inferno hosted on some other operating system, so that might was well be Plan 9. Then you can try them both.
