CS5 ACR opening PSD Files?

Question

What file types won't ACR open?
   I understand that ACR will not open layered images, but what about flat PSD files? When I Right Click on a PSD in bridge or mini bridge, open in camera raw is not an option. Command R, or file-->open with camera raw doesn't work either. Is it possible to open a PSD file with camera raw? From my tests 16 vs 8 bit  didn't make a difference, but should it?  Some of my friends have had trouble opening up Tiff files also. I know their preferences were correct.
Should Non layered tiff and PSD files open in ACR? What file types will ACR refuse to open?
And for extra credit...If a layered file opens in ACR are you only editing the background layer?
Thanks brilliant people of the internet!

Answer 1

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JimHess wrote:
Other than raw images (including DNG images), ACR will only open JPEG and TIF files.
Not true.
You can open .PSD files with ACR.  From within Photoshop, choose File - Open As - Camera Raw, and note that .PSD files are being shown with the File Open dialog filter in force.
You have to have saved the PSD with the Maximize PSD and PSB File Compatibility setting so that Camera Raw can read the flattened composite preview that's saved in that mode.  Otherwise it will complain that the file-format module could not parse the file.  Camera raw will only open the composite.
PSD files saved with Maximize Compatibility are of course larger because of the flattened composite being included, but as the option says they're also more compatible.
-Noel