I read a post in DotNetMafia(Corey Roth) that describes a stupid error in Infopath Form Templates in Visual Studio 2008 with TFS. 

 "As you are working, you might come to a point where you receive an error like the following when trying to save your file.

InfoPath cannot save the following form: InfoPath Form Template.  The form is read-only.

When this occurs, it really is a bad time because every time I have received it, the form was already checked out and not marked read only.  The only solution I have found for this is to close Visual Studio and restart it.  This can be particularly frustrating when you have made a bunch of changes to the form template as you will end up losing them.  For this reason, I have made a practice of attempting to save the form, before I start making any changes to it."

 You can read the full post via this link