"I would center the mouse pointer on the crosshair: right now you can't reach the right side and the bottom side, because as soon as the mouse pointer leaves the Silverlight surface the magnifying glass stops moving."
You will have to take my word for it that this isn't a problem - the alternative (center on the crosshairs) was where I started, but I found that your finger just got in the way!
Admitadly it wouldn't be difficult to fix, but I would have to lose some of the image (or make it less than full screen), which I didn't want to do.
Thanks for the feedback. (I will be buying everyone a virtual cake at the end of this)
You could rotate the magnifying glass according to the position on the screen. so if you are on the top, bottom, left or right you would always have the magnifying glass facing to the outside edge.
Also allow some single finger or single mouse action by using the double click.