Support for Mountain Lion is targetted for our 10.2.2 release that will be coming out sometime during the end of fall 2012. But we are working to get it included in a maintenance pack for support prior to that release date once we have all the code changes needed to support it.
There is a blog on this, I would follow that to stay current.
https://www-secure.symantec.com/connect/blogs/attention-mac-os-x-users-using-pgp-wde-or-see-fde-and-imminent-release-os-x-108-mountain-lion
UPDATE AS OF 8/3/2012 - the bugs that are tracking the Mountain Lion compatability are being actively worked by Engineering and I think we are in line with getting a compatible release within a month.