It seems like this piece would be named "Octagonal Dish 2 x 2" but I can't seem to find it on any of the usual sites. Is this named something else or is it not a real LEGO element?
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I believe it's yet another MEGABLOKS part. You can see it on the top of 9321 Cyborian rover holding the yellow antenna, for example.
I'm going with "This is not a LEGO piece", assuming completeness of this Brickset page: http://www.brickset.com/parts/browse/categories/?category=Plates,%20Special%20Circles%20And%20Angles which seems to be where it would be if it existed in genuine LEGO...
It is very obvious that it is a Mega Blok.