I wouldn't do exactly this. In my experience, Linux (specifically CentOS 3/4/5) is a generally poor choice for a NFS server. I have had several and found that under load, latency and throughput tend to drop for reasons we could never quite get our heads around.
In our cases, we were comparing back-to-back Linux's performance to Solaris (on Ultra-SPARC) and NetApp; both of which returned results in terms of apples-to-apples performance and in nebulous terms of "engineers not complaining nearly as much about latency when the server was under load". There were multiple attempts to tune the Linux NFS server; both the NetApps and Solaris systems ran as-is out of the box. And since both the Solaris and NetApp systems involved were older, the Linux servers could be argued to have had every advantage and still failed to be convincing.
If you have the time, it would be a worth while experiment to set up the same hardware with OpenSolaris (now that Solaris is effectively too expensive to use), Linux, and perhaps a BSD variant or two, and race them. If you can come up with some performance metrics (disk I/O counts in a VM hosted off the store, for example) it might make for an interesting white paper or internet article. (If you have the time.)
Regarding NFS in general, the NetApp people told me several times that their benchmarks showed NFS only had a cost 5 to 10% in performance for VMs -- and if your application was sensitive enough that this was a problem, you shouldn't be virtualizing it in the first place.
But I should confess that after all that time and tears, our non-local production VM stores are all fed by iSCSI, mostly from NetApp.