A major drawback with iPhoto has always been the fact that it can’t gracefully handle multiple libraries. Albeit a hack, iPhoto 2 fixes this limitation by adding limited support for multiple libraries. Although there is no menu option for switching between libraries there are a few different techniques that can be used.
The best method that I’ve found is using iPhoto Library Manager to manage and track multiple libraries. The software compiles a list of available libraries and allows you to effortlessly switch between them.
The simplest method that I ran across involves dragging (or Command-Option dragging) an iPhoto Library folder onto iPhoto. The most recently used library becomes the default.
The award for the most inelegant hack involves storing iPhoto libraries on encrypted disk images. I can see this extremely beneficial for the porn addicts.