Does anyone know how to tweak ST2 / SideBarEnhancements (or some other setting) to get .psd files to open in Photoshop CS5.1 ?
First, you need to enable viewing of .psd files in the sidebar, as they're hidden by default. Add the following line to your user preferences (Preferences -> Settings - User):
"file_exclude_patterns": ["*.pyc", "*.pyo", "*.exe", "*.dll", "*.obj","*.o", "*.a", "*.lib", "*.so", "*.dylib", "*.ncb", "*.sdf", "*.suo", "*.idb", ".DS_Store", "*.class", "*.psd", "*.db"],
Next, right-click on a .psd file in the sidebar and select Open With -> Edit Applications.
If you're on OS X, edit "application" to point to the app name, not the full path. If you're on Windows, put in the full path to the Photoshop .exe file, using forward slashes / as path delimiters (for example, C:/Program Files/Adobe/CS5.1/PhotoShop CS5.1/photoshop.exe, or whatever it is on your system). You can also edit the "caption" field to make it more descriptive if you want.
Save the file, and you should now be able to right-click .psd, .png, .jpg, and .jpeg files and select Open With -> Photoshop.
Good luck!