Update: As of 2.75 this data is saved for all renders.
This info is written to the header of the file and only for auto saved renders (animations and command-line renders). For info on how to read the headers see the documentation for the relevant format. I don't believe GIMP has a way to view this data. As ideasman42 said, for EXR files you can use exrheader which is included in the OpenEXR package, for PNG and JPEG files you can use identify -verbose command from the ImageMagick package.
