Shameless plug: a few friends and myself ported the reference toolchain to Solaris (illumos). We hope to integrate this upstream now that Go 1.2 is released.
It is not illumos-specific, but currently it does not work on Solaris 10. It would be trivial to make it work on Solaris 8-10. The main reason I haven't fixed it is because I don't have any machine running Solaris 8-10. We need a builder that does continuous integration.
If people are interested in these older systems, and if someone donates a builder, I can make Solaris 8-10 work as well.