I didn't notice you had the Question published here as well (ref. thread on Silverlight.net : http://forums.silverlight.net/forums/p/107493/330885.aspx#330885)
Copy of answer from there:
this is a complete oversight on my behalf in the Engine.
Sorry about that.
Over the last couple of months a lot has been refactored and apparently, the Actor system has been forgotten a bit. I will put this up as a bug on our taskboard. One thing though you could do to overcome the problem, for now at least; when you create the
Actor from your Game class, you could add a dependency on your constructor to IContentManager and pass along the one you have in the Game class and use that. This is of-course something that will be fixed and handled automatically, so you don't have to worry
I've put it on GitHub issues list that we use for work: