We haven't seen this issue for several months. We've done several updates/reconfigurations so I'm not sure what (if anything) fixed it.
What we did was:
-Ensured chassis and individual blades are at the latest firmware
-Configured Virtual Connect to use VLan tunneling mode
-Limited number of "active" NIC's on each vswitch. Set redundant NIC's as "standby"
-Network group updated firmware on 6500
Hopefully we'll never see the ARP issue again...