Ok, so you learn something every day.
Currently I’m working on a project where I have a number of separate component systems. In particular there is suite of services exposed over HTTP that are consumed by a couple of other projects. One of these is an ASP.NET application, both the services and the ASP.NET apps are using Cassini as their web server for dev.
I was having trouble when I wanted to debug both the client and the service, I’d hit F5 and run up the web site, and discover that the web server for the services wasn’t running too.
However this is easily overcome by right clicking on the web services project Debug > Start New Instance