If you have set up Cassini to start with a custom port number which passes the current project directory to Cassini as a command argument from External Tools, it is now broken after installing SP1. Turns out SP1 breaks the way the project directory is passed, they apparently forgot to preface it with a double-quote (").

In your arguments, change it

From: /port:8080 /path:$(ProjectDir)

To: /port:8080 /path:"$(ProjectDir)

(Note the double-quote between "/path:" and "$(ProjectDir)")