How can I stop accidentally checking in a file in TFS

I have a web.config that occasionally I'll checkout and modify to use a hardcoded password instead of reading it from the registry.

How can I prevent myself from absent-mindedly checking it back in and annoying my co-workers?

I could achieve this in the past with Perforce by creating a separate pending changelist.

edit - I'd rather not make it writable, I was wondering if there's anything in TFS or planned for TFS to support this?