Photoshop CS6 13.0.1 x64 (Windows 7)
If you open a document, change the resolution to something different from the original then use Save For Web (JPEG) selecting All Metadata, the new document is saved with the original PPI not the new one.
Sorry to say but the bug exists in Photoshop CS5 as well
Photoshop CS4 still has the original PPI in the raw data but reports that it is 72PPI in Photoshop.
So it's been a problem since CS4 and no-one has noticed?