iPhoto "passworded folder" mechanism
One option would be to have multiple iPhoto libraries, and keep one on an encrypted Disk Image.


Create an encrypted disk image in Disk Utility, or some other utility. This might be a good use for Knox, which lets you easily create an encrypted disk image and unlock it and mount it when you want.
When you open iPhoto, hold down the ⌥ (Option) key and you will be asked to select which iPhoto library you want to open.