I think I may have just run into a real live Heisenbug.
This showed up on Mac OS X 10.37 and Entourage 11.10. A user called, complaining that a batch of jpegs dragged from the attachment section of an email to Preview.app in the Dock opened in Photoshop instead of in a single window in Preview. He showed me what happened. In his case, the jpegs were tagged as Photoshop files so that makes a certain amount of sense, although I would expect preview to operate through the Dock as it does in the Finder - opening any image it can handle.
The part that does not make sense, is that dragging from Entourage to the Dock isn't supposed to work. Neither I nor a colleague can get it to do that on our machines, and it has now stopped working for the original user. I've told him not to mention it to anyone.
If anyone out there has a Mac with a patched-up Office 2004 running Panther, try it and let me know.