There is some kind of incompatibility between Linux 2.6 NFSv4 server nad Solaris 10 (U3) NFSv4 client. On installed Solaris you can put some variables into /etc/default/nfs and it should work, but when you are trying to bootstrap from Linux NFS server using Jumpstart you have to search for another solution:
1) Build a new miniroot image with /etc/default/nfs altered?
2) Simpler… alter Linux NFS server to provide only eg. only NFSv2 service
This can be achieved by recompiling kernel without NFSv4 or by much more cleaner solution – disabling NFSv4 services on runtime.
Place the following in /etc/sysconfig/nfs (RHEL5/CentOS5 specific configuration file):
RPCMOUNTDOPTS="--no-tcp --no-nfs-version 4 --no-nfs-version 3"
RPCNFSDARGS="--no-tcp --no-nfs-version 4 --no-nfs-version 3"
Now execute
/etc/init.d/nfs restart
That’s all! 
Jumpstart problem solved!
For more info consider reading man pages for rpc.nfsd and rpc.mountd. Internally those switches write “+2 -3 -4″ to /proc/fs/nfsd/versions. Versions file can be only modified after stopping [nfsd] kernel service ( you’ll get EBUSY errno while trying to change it with nfsd lanuched ).