The VMware community is buzzing about problems when using NFS datastores on NetApp filers with ESXi 5. Here's the link to VMware's KB article that describes the problem and resolution in detail.
If you're able to, enable Storage I/O Control on your filer, and you won't need to worry about the issue. Upgrading to a more recent version of NetApp's ONTAP operating system will also address the problem. Or you can modify the nfs.MaxQueueDepth advanced setting in ESXi.
In any event, if you're doing NFS to NetApp for your ESXi 5 datastore, please read through the article and take the appropriate action.
Here's Cormac Hogan's post that raised awareness on the issue. If you're not reading his blog, you're missing out on tons of great posts about storage and virtualization.