Trust user for delegation: AD access denied

[pierrot.heritier@unifr.ch]

Hello all
I'm trying to setup Kerberos on my Windows 2003 domain. I already had
to raise the domain functional level to Windows 2003 in order to get
the Delegation tab in the SQLservice account. 
Now, when I try to "trust this user  for delegation to any service
(Kerberos only)", I get an Access Denied from the Active Directoy,
although I'm logged in as domain admin.
I suppose I'm missing something somewhere, but what ?
Pierrot