...The program quite rightly chooses Photoshop CS5 as the editor but refuses to open in CS5 stating that an external editor isn't selected. I checked preferences and it is set as default. When I choose the second option in preferences I can "force" it to open in CS5. This however means that the second option can't open an other program. Posted in the Adobe site,so far not a definitive answer.
You can set up any number of editors, or, for that matter, a program such as Qimage, as a second option and then save it. I right-click on an image, select Edit In, and get a list of all the programs I might choose to have the image open in. The return to LR is automatic, and the image gets automatically imported back into LR.