In Samba 3.2.5 and Samba 3.4.7 doing net ads keytab create used to create entry for SPN host/fqdn.
In samba 3.5.0rc3 and 3.5.2 I don't have this entry anymore (but others like host/server and host/netbiosname are ok).
I tried to tweak by using this:
* secrets only
* secrets and keytab
But it seems to be without results.
After some investigation in the code it seems that the problem is due to the fact that dnshostname sometime didn't contain the fqdn.