As of today, it is not.  If you haven't had your sales team file an FR for it, that's a good idea.  Isilon is aware of the feature request, but the more customers that log it, the better the chance of it getting priority.

In the mean time, most customers are more concerned with traffic that leaves the site and use some sort of VPN between their sites to encrypt all traffic.  If the clusters are within a building/site, sometimes a private replication VLAN is used.

Thanks Adam,

Is there a place/document where this is called out?

Not that I'm aware of. It's unusual to document things a product does not do as that could be a really big document.