Yes, you can create and manage multiple iPhoto libraries by holding the Option key and starting iPhoto, but iPhoto Library Manager is waaaay cooler. Multiple libraries, quick view, sharing, sync with iPod, copy photos between libraries, split libraries, merge libraries.

iPhoto Library Manager adds an additional level of control over your iPhoto collection by allowing you to keep your photos in multiple libraries and transfer them back and forth without losing your keywords, ratings, and all the other information you’ve worked to assign to your photos.

9WR - iPhoto Library Manager: Manage, switch, merge, copy photos, sync, share multiple libraries.