Comment # 3 on bug 1179629 from Fabian Vogt

(In reply to Maurizio Galli from comment #2)
> (In reply to Fabian Vogt from comment #1)
> > What's the output of "xauth list"? It should contain an entry with "#ffff#".
> 
> Right after fresh install:
> 
> mau@localhost:~> xauth list
> localhost/unix:0  MIT-MAGIC-COOKIE-1  d56f411b012a955974a66232540e958e
> mau@localhost:~>

The FamilyWild entry is missing, so most likely a bug in GDM.