I came up against an issue mounting via nfs from a Linux host to an OpenSolaris virtualbox guest (see this post for automounting nfs on OpenSolaris). Checking the host's logs revealed the issue: mountd:  [...] refused mount request [..] illegal port.

This is seemingly caused due to the natted connection between guest and host. Adding (insecure) in the host's /etc/exports line for this export enabled the guest to successfully mount the nfs share.

The specs are here for this machine. I am currently running Ubuntu 9.04, the development version of Ubuntu. I also run virtualbox on it with Opensolaris 2008.06 as the guest O/S ... I haven't had a chance to work out some issues precluding installation of OSOL as a dual-boot option.