Its quite simple to setup CI with Cruise Control.Net and CodePlex. All you need to do is use SVN to pull down the latest code using the Exec task as shown below, then build the solution. This example pulls the code from the DNN Automation Tests project. SVNClientPath is the path to the svn.exe on your build server.
<Exec Command="%22$(SVNClientPath)svn%22 checkout https://dotnetnukeautomation.svn.codeplex.com/svn C:\Tests\Source" />
You will probably need to change some config values before you build. To do that you can sue the MSBuild Community Tasks project and use the XmlUpdate task as shown:
Once you have the code and have done your updates you execute the build as follows.
<MSBuild Projects="$(TestsFolder)\Source\DotNetNuke_WatiN_Tests.sln" Properties="Configuration=Release;Platform=Any CPU" Targets="Rebuild" />
Now just create a CCNet project and call the task that has the above two steps and you're done.