So, got the oft-reported error just now. Still using CS4, until Monday or Tuesday.
The file was a placed image in InDesign, and opened into Photoshop using the Edit Original feature. Made my changes, hit Save, got the "file in use" error.
Used What's Keeping Me to see what process was preventing me from saving the image... drum roll please... it's InDesign.
So, how to use the Edit Original feature if you can't save your work to update the link when you're finished???