[CentOS] CentOS 6 and XDMCP

Hi all,

I am trying to use XDMCP from a Mac (OSX Lion, XQuartz) to an updated 
CentOS 6 64Bit host.

I am tunneling through SSH, so I use XDMCP on the remote host locally 
via Xnest like this:
> ssh -X -C -l $LOGINUSER $HOST -n "Xnest :1 -ac -geometry 1440x850 
> -once -query localhost
I get a black screen without any login window or desktop background.

My /etc/gdm/custom.conf looks like this:
> # GDM configuration storage
> [daemon]
> [security]
> DisallowTCP=false
> AllowRemoteRoot=true
> [xdmcp]
> Enable=true
> [greeter]
> IncludeAll=true
> [chooser]
> [debug]
According to the howtos on the net this should be sufficient, so I am 
quite stuck. How can I figure out if this is a problem of the CentOS 
config or of XQuartz?

Any hint or help would be appreciated.

Dirk