I received an error while trying to vMotion today: "A general system error occurred: Migration failed while copying data. Busy." The issue turned out to be a swap file change in 4.1. To get the machines to vMotion again I had to change an advanced setting: Migrate.VMotionResolveSwapType from 1 to 0. Then everything migrated after that.
Hopefully this will save someone the hour I spent with vmware tech support figuring this out.