Beginning with kernel 2.6.37, it has been changed the meaning of dropped packet count. Before, dropped packets was most likely due to an error. Now, the rx_dropped counter shows statistics for dropped frames because of:

Softnet backlog full
Bad / Unintended VLAN tags
Unknown / Unregistered protocols
IPv6 frames when the server is not configured for IPv6

If any frames meet those conditions, they are dropped before the protocol stack and the rx_dropped counter is incremented.
Additional Information
Care should be taken to confirm that frames are not being legitimately dropped.  A quick way to test this (WARNING: this test does not work for bonding interfaces) is to start a packet capture:

host:~# tcpdump

And then watching the rx_dropped counter.  If it stops incrementing while the tcpdump is running; then it is more than likely showing drops because of the reasons listed earlier.  If frames continue to be dropped while running tcpdump, investigation should take place to determine root cause.