The amount of effort to make your application compatible with Cassini in many cases will be Zero. Cassini is reusing bulk of the technology that makes ASP.NET working in IIS. If your application does not rely on IIS-specific features, like server variables unique to IIS, then you are good to go. Try running your web application on Cassini and you'll be surprised by how easy it was. The biggest (even though still very small effort-wise) difference will probably be in how you create the installation package for your appliaction. Instead of making Web Installer project, you will have to make regular installer project, and register/unregister your application with Cassini as a custom installation step.

Applications as complex as DotNetNuke can run on Cassini. This means just about any properly written ASP.NET application can to run under UltiDev Cassini web server.

Screenshot of DotNetNuke running
under UltiDev Cassini web server: