I've setup an NFSv4 server (Debian) and client (Ubuntu/Mythbuntu). It seems that uid username mapping is working nicely out-of-the-box (I get correct usernames on ls -l if the usernames match between the two boxes, even if the uids don't), but ACLs are not working. I've installed nfs4-acl-tools and I can read the ACLs correctly on the client, but they don't get applied.

What needs to be done for ACLs to work?

To clarify; username mapping works for regular permissions. ACLs are applied using uid/gid (I can even set ACLs by uid and they work).