Mono is probably the best choice if you're primarily a .NET developer and don't care very much about making a well-integrated Mac app (one that behaves like a Mac app in all the many ways Mac users expect). Given your problem space, that sounds like where you are. That said, pay special attention to this limitation from the WebBrowser page if it applies to your problem:
The Mono Webbrowser currently lacks
support for window.external, the
bridge that allows Javascript code
embedded in the browser to communicate
with the managed world using the
ObjectForScripting.
QT is very well established, and a great platform if you want to develop this in C++. Remember that there are licensing fees if this is a commercial app.
I've played a bit with wxWidgets. It's big advantage here is that it's probably going to be the smallest distribution, since you only have to include what you need. If all you need is the web browser, I'd look at this, but in my experience it's hard to make wxWidgets programs look good or act correctly on Mac (or on Windows for that matter).