I taught both iPhoto 08 and Aperture classes today on the Geek Cruise, and one of the key points that resonated with students was how beautifully iPhoto can serve as a deluxe plug-in for Aperture.
The basic workflow goes something like this: Use Aperture as your main database for uploading, organizing, rating, keywording, and editing your pictures. In its Preferences pane, turn on previews and sharing with other applications. Then, when you want to use iPhoto's cool slideshow function, make greeting cards, create calendars, publish web galleries -- access your Aperture images using the File > Show Aperture Library command. You'll see a pop-up dialog box as illustrated here. Drag the photos you want into iPhoto (it will use the previews you generated in Aperture, so the better the previews, the more options you have in iPhoto.)
In this manner, iPhoto becomes a fairly extravagant plug-in for Aperture. This can be especially nice for Raw shooters who want Aperture's horsepower but the output goodies in iPhoto.