What is the benefit of moving IE9 to the CLR? You asked why it isn't, but you never really said why it SHOULD?
If your reasoning is security then frankly IE9 could do more good by dropping back-compatibility with every existing addon and OS tie-in, and only allow back in those that supported low privileged execution. If they developed a CLR version and supported existing addons etc then it would do absolutely no good at all.
Frankly I'd like to see all browsers move to a "whitelist" model for automatic file type execution... Drop PDF, Office Formats, and unknown formats entirely from auto-starting and require the user download, save, and run them.
I did...touch on the benefits of writing IE on the CLR earlier in the thread...
if IE were written on the CLR we would get a lot of benefits. The innovation in the browser would drive the evolution of the CLR and vice versa.
here is an example of the kind of things we would get; when the browser folks need to create the concept of isolation so that tabs would not crash each other for example. We could use the same technology in our .Net apps.
The browser folks would benefit from any good work that was done on the CLR; new techniques to code for concurrency is a good example.
there are many more benefits...