Ian,
Did you even update to a newer version of Silverlight viewer and find that it did not work anymore? I went from 3.0 to 3.3 and the measure tool no longer works. I am trying to modify it in Visual Studio but it says my Silverlight version is not high enough. I'm at version 4. If I download 5 it tells me it is already installed. Long story short, you wouldn't happen to have a .xap file that works with 3.3 would you?
This add-in has the measuring units defaulted to feet. I worked on this file back in October. Let me know if this works.