pptPlex looks good... But it seems limited feature and limited support.
I have been using picLens ( http://www.cooliris.com/ ) for a while for helping our sales team with presentation. The effects are quite good. It supports PP 2003 and Keynote. Has IE/firefox plugins.
I think MS should make pptFlex full featured so that it can become competitive to products like picLens and will help in promoting products like touchwall.