I may have spoke too soon when I said that we weren't having the issue any more. Earlier today I found one VM that was unable to ping a different server. Both in the same subnet, but on different blades. I could ping from the target to the source, and then ping from source to target worked OK for a while, then stopped working again. I vmotioned the target to a blade in a different chassis and everything worked OK.
I can't find it on anything else, but that doesn't mean it's not happening. We have something like 300 servers, and most of the communication is NOT server-server , so short of having every server test ping every other server, it's not really going to show up..
NV1, you mentioned that a workaround was to use active/standby NIC's. Do you mean to have on one NIC on the virtual switch as active and the other on standby? We are doing ours with all the NIC's on the virtual switch as active. I'd really hate to lose the 10Gb bandwidth by turning one NIC as standby..