Consider painting the formatted documentation in a compact form above the member like the sample CR_XmlDocCommentPainter that ships with CodeRush.
The CodeRush one is nice but does not handle multi-formats within the details (like <code/>). This should be available as an option or shortcut.

I'm going to have to think about how this would work. Particularly on the class- level documentation, rendering inline could get really big - bigger than the actual XML doc in some cases.

I'm also not sure what would happen when you click in it - would it switch to the XML so you could edit it? Would it stay rendered?

Unfortunately, I think the ability to edit the doc while rendered is outside the scope of what I'll be able to deliver. I've done some spikes on that and... it's... hard. :S

The more I think about this, the more I wonder if it's something we want to do. A full doc preview - the goal of the plugin - is a bit large to put inline.

A better goal might be to draw an icon next to each XML comment block that allows you to display the doc preview if you don't already have the CR_Documentor window open.