When the TFS Services preview went live, I started moving my on premise projects to the hosted tfs services.
At some point I wanted to delete a team project created in the hosted services. For now, the web interface does not provide an option for that. However, the commandline tool tfsdeleteproject works fine.
tfsdeleteproject /force /collection:https://[yourname].visualstudio.com/DefaultCollection [yourproject]
Warning: This operation cannot be undone!