+1  A: 

Changing the mask adding lines to:

canvasSprite.addChild( maskSp );
canvasSprite.mask = maskSp;

Solves the problem, so there is likely some confusion with the relative coordinate spaces of the clips. I think you should keep your mask on the same level as your masked DisplayObject in your displaylist to avoid confusion.

grapefrukt
Thx! I modeled my code after the mask example in the api, which appears to be a little buggy when modified as in my example.
jedierikb