My N36L running ESXi and Ubuntu Server 11.04 as a VM maxes out one core @ 85-90MB/s over SMB. Seems that smbd isn't multithreaded?
With Jumbo Frames I can squeeze out a few more MB/s.
Take a look at http://www.samba.org/samba/docs/man/Samba-Developers-Guide/architecture.html