Hey,
I believe SunNFS now defaults to using Kerberos security instead of the older system ip/uid/gid based authentication.
The new mount option is 'sec=' and you'll probably be needing 'sec=krb5'.
If you want to continue to use the old scheme you'll need to change the exports on the server to explicitly 'sec=sys' or you can use 'sec=krb5:sys' to allow both.
The newer man pages for both 'exports' and 'nfs' outline the options.
A quick google came up with this presentation which goes through the changes from the old to new SunNFS authentication:
www.connectathon.org/talks96/auth.pdf