Over the weekend I finally managed to get 10GbE in a Mac and test network filesystem speed.
At this point, after considerable client-side tuning, sequential NFS performance is in the 230-250MB/s range for both read and write. I haven't yet tested SMB or AFP performance. I would not expect either to be faster than NFS.
The client is a current-generation Xserve with 2 Xeon 5620's and 3GB RAM running OSX Server 10.6.4.