Thin vmdk files are supported on NFS datastores as of ESX 3.0. From the service console you can compare the output of the 'ls' command with the output of the 'du' command to tell if the vmdk is thin provisioned.
For example, this file is defined as 20GB, but thin provisioned and only using 92MB:
[root@esx ericXP]# ls -lh ericXP_1-flat.vmdk
-rw-r--r-- 1 root root 20G Mar 26 16:10 ericXP_1-flat.vmdk
[root@esx ericXP]# du -h ericXP_1-flat.vmdk
A slightly more efficient way of doing this is to simply add the '-s' option to the 'ls' command:
[root@esx ericXP]# ls -lhs ericXP_1-flat.vmdk
92M -rw-r--r-- 1 root root 20G Mar 26 16:10 ericXP_1-flat.vmdk
In all cases, you want to look at the -flat.vmdk portion of the file pair.
You can also determine this from the VI Client (as of ESX 3.5) by comparing the "Capacity" of the "Hard Drive" in the Virtual Machine Properties window with the Size of the file reported by the Datastore Browser.