As mentioned in the link below...
ExCanvas does not support any of the
ImageData functions (createImageData,
getImageData, putImageData). Given
the way it's implemented it's highly
unlikely that we'll be able to add
support for it in the VML
implementation of ExCanvas.
It should
be doable in the Silverlight version
but we haven't spent much time on that
lately. ExCanvas supports most of the
functionality as specified in the
initial apple canvas spec but few of
the additions added by the gecko
implementation and later speced by
html5 working group.
The reason being
fairly simple, ExCanvas uses VML to do
the actual drawing and many of the
additions (such as getImageData)
requires pixel level access which
isn't provided by VML. Your best bet
for now would be to keep track of all
commands used to draw the canvas and
use those to generate an image server
side.
http://groups.google.com/group/google-excanvas/browse_thread/thread/7d35fa72dbe1487b
Why can't IE just use a real canvas like all the other kids....