Two thoughts: First of all, I'm a Python fan, and would like to use it client-side. But the truth is that ECMAScript ain't that bad, it's just a little strange. I'm not at all convinced that it's a huge win to use Python over ECMAScript, although I'd choose to, given the opportunity.

Secondly, this is cool, but if it depends on users installing a plug-in to IE for anything to happen, it's unlikely to amount to much. I don't see devs using anything (except Flash) that forces their users to choose between installing something (scary!) or going to another site.