Take a look here where the difference between WGS84 and various other "observations" is documented at various locations along the prime meridian

http://tinyurl.com/76jv4

From: BoundaryPoint@yahoogroups.com [mailto:BoundaryPoint@yahoogroups.com] On Behalf Of Roger McCutcheon
Sent: Tuesday, January 03, 2006 6:20 AM
To: BoundaryPoint@yahoogroups.com
Subject: Re: [BoundaryPoint] Four pointer

I went to maximum magnification on Google Earth and found that it did indeed show zero longitude as being about three quarters of the way to the far side of Black Heath Avenue, to the east of the Zero Meridian in the Royal Observatory, so then I went to the Prime Meridian site and found that "the zero meridian on the WSG84 datum, which is about 100 metres to the east of the line marked at Greenwich, is an average of the various continental movements", so we need not worry: someone is paying attention! Roger McCutcheon.