Will a cluster failover occur if a NIC card fails in one of the
clustered
filers?
No. Generally speaking, cluster failovers occur on "catastrophic" failure events involving the systems core electronics, most of which are located on the system motherboard (processors, memory, core supporting circuitry, that type of thing). Individual I/O adapters such as NIC cards are not really "in the core", but can be protected from failure events via other means, even means that do not require you to go to the additional expense of implementing a full cluster.
For example, "simple" NIC failover capability has been introduced in Data ONTAP 5.3 release ("simple" in that you don't even need to do the full Etherchannel trunking thing anymore to get it! :-)). See:
http://now.netapp.com/knowledge/docs/ontap/rel531/html/sag/net14.htm#1186750
for specific details, or more generally to all the sections on interface configuration in chapter 4 of the 5.3 System Administrators Guide.
Keith