I know this topic is kinda old, but it is one of the top hits on google search for RDM and vMotion:
http://www.vmware.com/community/message.jspa?messageID=313145
Anyway, to get around this limitation we are using iSCSI software initiator's on our Red Hat guests running Oracle OCFS2 and accessing shared iSCSI luns (instead of shared RDM's).
vMotion seems to work fine in our tests. But we haven't pushed it out to any of our more important servers (dare I say production) or done extreme IO load testing.
Anybody else doing this successfully? Any caveats or gotchas?