What is the configuration of your virtual switche(s) on the ESX 3.5 server?
Since you can vmkping, I assume you have a vmkernel port. On the same virtual switch with the vmkernel port, you will need a service console port for iSCSI. This is how the ESX box establishes communication with the NetApp filer. If your vmkernel port is on a different virtual switch than your service concole (best practice), then you will need a second one.
Something like this (this is all from memory so it should be close):
First Virtual Switch:
Service Console Port with an IP address (for example 10.10.10.1)
Second Vrtual Swtich:
VMKernel Port on iSCSI network (10.10.20.1)
Service Console #2 (10.10.20.2)
The second service console isn't needed for NFS on ESX 3.5, just iSCSI. With vSphere, I have heard it isn't needed anymore but I haven't tested it.