MBR not overwritable with dd?

[Emanuel Strobl]

I tried to null out the MBR with the BETA7 fixit CD with the follwoing 
command:
dd if=/dev/zero of=/dev/ad0 count=16

After that fdsik still showed me a valid partition tabel!
How? Does GEOM map the beginning of the raw device?

Thank you in advance,

-Mano
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