(I'm running Windows XP)
When you "Select an Area" in the internal Preview frame (or Split window), I'd like to be able see "Highlighted" (selected) the text in the source code that is producing it. The reverse should also happen, when I select text in the source code, I should be able to see where in the Preview frame that code is being rendered, it should be highlighted (shaded, or outlined).
I realize this requires different functionality than a simple browser instance for preview, but it is a feature I've come to expect from a web development tool. (see Dreamweaver, Kompozer, etc.) The point of having an IDE is to have it "all" integrated into one application. I'd rather not have to run Kompozer just so I can correlate code to it's rendered area.
I'll be interested to hear what your comments are about this. Or, is this already on the 'to do' list for the 2010 version?

Please also refer to my 14 Ultimate Feature Requests.