A while back, I accidentally found a very easy way to switch between iPhoto Library directories with iPhoto2. All you have to do is drag the new iPhoto Library folder onto the iPhoto icon in your dock, remembering to press and hold Command and Option before you release the mouse button.

If iPhoto is not running, it will start up right away with your new iPhoto Library. If it is already running, it will first close the current iPhoto Library, and then open the newly selected one.

The key to this trick, obviously, is the dock's "force open" capability, invoked by holding Command and Option when dropping on a dock icon. I'm always surprised at how little PR this functionality receives.

[robg adds: Another means of switching the library is discussed in this hint.]