The web browser control in .NET is just the native code rendering component. If there is a compelling reason to write one in .NET then why not start a project on CodePlex today?
FWIW I think there are advantages to having a webbrowser written entirely in WPF. Not necessecarily because of "security", "plugins" or "speed" but because it would open up the possiblity of applying transforms and other WPF-y effects that you can't
do on an interop'd control. That would be a far more compelling argument to me. I'm also not naive enough to believe that such a thing would be trivial (though I'd certainly be interested in helping if there was a project underway!)