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.