I don't suppose anyone is any closer to getting a solution to this. I am experiencing exactly the same problems.
I have found that if I have entities within an Xref that are set to a BYLAYER colour and the layer colour is a True colour (for example 226,242,248) then when inserted it displays as Black. If the entity is set directly as the true colour value rather than BYLAYER then it is displayed in the correct colour.
If I bind the Xref or in-place Refedit then the colours also suddenly become correct and if I print the drawing then the output is also correct so it only seems to be a display issue with the xreffed drawing. This is all when I am in Model space.
The problem doesn't occur on a different PC in our office so it appears to be specific to something in my set-up.
I've tried various settings for my graphics card (NVIDIA Quadro FX1600M), made sure I've got the latest drivers etc. but still seem unable to stop the problem occuring.
I've recently installed AutoCAD 2011 and was very pleased to find that the problem doesn't exist there, so hopefully it's something Autodesk have now fixed in the new release and will filter back down to 2010 (eventually...)