Lewis Harvey posted a great fix for the "black shadow" issue in transparent tile overlays on Virtual Earth - something that has been plaguing our platform since we began natively supporting tile overlays. The fix was completed by the dev team, tested and ready for release to production, but other priorities have prevailed. This is a nice workaround for those of you struggling with this issue.
I was actually able to attached the function to the onLoadMap property and it worked when the map loaded. You'll also want to attached it to the onchangemapview as Lewis recommends.