CS5 won't apply edits in ACR

There is a setting in the ACR preferences (you can get to this from PS prefs, Camera Raw section) as to whether develop changes are stored in external XMP, or in the Camera Raw database (which AFAIK, equates to Bridge just remembering a copy of this, somewhat as Lightroom does - but these two types of database are invisible to each other, as opposed to external XMP which is fully visible).

Speculation: in cases where an XMP sidecar or embedded XMP already exists on the file, this may be taking precedence when the image is returned to, over the information which has been written into the ACR records. I don't know if this is quite what is happening; but it seems to fit the symptoms at least. So it is IMO worth first checking to see how/where your ACR develop changes are currently set to be written.

Also I'd check to see whether you have "write" permissions to that folder. If you don't, the external XMP sidecar cannot be updated.

RP