Silverlight isWindowless (Transparent background of the canvas)
I made this mistake earlier so wanted to share my problem and solution with you.
Watching a Video of a session from MIX '07 that Mike Harsh presented and he walks through creating a transparent canvas, except copying his code just didn't work: -

What I needed to do rather was this (note one's a string and the other is an embedded boolean - dunno why, but that seemed to have sorted it): -

We've got MIX UK coming up in a couple of weeks, should be lots of cool Silverlight stuff shown off there :)