How will I know what port Cassini will run on for Watin testing?

Well, you can set this for a project. Right-click on the project in the Solution Explorer in Visual Studio 2010 and select Properties from the menu that appears. In the project's properties, navigate to the "Web" tab (for a web project, duh) and click the "Specific port" radio button. You may need to run your web app once to get it to disassociate with the port it was using beforehand before the WatiN stuff will work.