In Team System when you are working in Solution Explorer and you create new or add existing files, the file is created on your local workspace and is added to Source Control when you check your changeset in.
Working in the Team Explorer has the opposite paradigm, however.
When you add a new file in Team Explorer (such as new build definition), it is created in the source repository, and your local workspace does not have access to it until you perform a Get Latest. It shows up in source control grayed out and you cannot perform any action on it such as checking the file out for edit:
Getting the Latest will create the file(s) in your local workspace and you can then work with them.