Ok; I've tested against the same server that Etienne was testing against, and found the particular error reproducible; also from my own client, where I was getting the 'Required key not available' message instead of the 'Function not implemented' error. I think the 'not implemented' is associated with this output in dmesg:
[771815.562213] CIFS VFS: secType 5 not supported!
[771815.562368] CIFS VFS: Send error in SessSetup = -38
[771815.701453] CIFS VFS: cifs_mount failed w/return code = -38
So that's a server configuration problem, not an issue on the client side; the server in this case is actually not joined to an AD realm at all, TTBOMK.
If I instead point at a win2k8 server that's part of an AD realm, I get the following in dmesg if I use sec=krb5:
[ 5818.652927] CIFS VFS: Server requires packet signing to be enabled in /proc/fs/cifs/SecurityFlags.
[ 5818.782901] CIFS VFS: cifs_mount failed w/return code = -95
And if I use sec=krb5i, I'm able to mount successfully.
So it looks to me like the packages in PPA work as intended, provided that the server you're connecting to can accept kerberos connections.