Apparently it is a known problem for OS data to not line up with Bing Maps service and in order to get round this you have to re-project all your British National Grid data to the WGS 1984 Web Mercator (Auxiliary Sphere) coordinate system. How rubbish is that!
Anyway this problem is apparently known to ESRI and it is apparently fixed in verison 10 (it was tested by ESRI support in a beta 10 version). So I guess we will have to wait until thats out and a few service packs have been released until we can feel confident with rolling out version 10.
On that note I’ll talk about this issue in about another year to 18 months (I would have thought!)