There is a huge new problem with the iPhoto library being a package instead of a folder. This involves the editing of photos previously imported into iPhoto 06 and previously edited in another application.
If I want to edit them in another application again (photoshop) these photos cannot (at least for me) be saved back into the iPhoto library. The help data says I can do this if I do a Revert to Original first, but it's definitely not working that way on my machine.
To reedit these images, I have to open them in Photoshop, save outside the iPhoto library, reimport to the iPhoto library as a new event, merge events, and then hide the original photo in iPhoto. In 06, all I had to do was save.
This does not appear to be a problem with images newly imported in 08, but my primary 06 library contains 21,228 images from my iPhoto 06 library. That's pretty ugly for something that is called an upgrade.
Other than this and the fact that 08 is dog slow compared to 06, the new features are really quite nice.
iLife '08: Part 1 - iPhoto - a Bounty of Surprises