Next you'll need my rotate90r.applescript AppleScript. Use Script Editor to save it as an application somewhere. It is my first piece of AppleScript, so be kind to it. If you drop one or more pictures on its icon, it will rotate the images 90 degrees clockwise in place. The AppleScript is written assuming a Fink installation of jpegtran; if you used DarwinPorts, you'll need to modify the set trancmd line to match your installation.
Then launch iPhoto and open its Preferences. There is a field called "Opens in other:". Select the AppleScript application you just saved. If you also select the "Opens in other:" button, then a double click will rotate the selected picture. Or you can Control-click picture and select "Edit in external editor." iPhoto will save the original picture when you rotate it the first time, so you can use "Revert to Original" menu to undo rotation.
[robg adds: I haven't tested this hint, but I looked at the AppleScript, and there's clearly nothing malicious in there...]

