You have all probably figured this out by now but Aperture 3's image zoom tools are pretty cool. As part of the SuperSimpleStuff™ series I wanted to note one minor behavior that you may not have noticed based on a one on one session that I did where this came up.
As you know there are about 9 different ways to zoom an image depending on if you have a track pad or not along with the built in keys and predefined zooms. One of the coolest ways is to drag the mouse over the "percent" label to the left or right on the full image preview thumbnail that pops up no matter what zoom mode you are using.
The question that came up is how to "get out" of zoom mode and get rid of that little thumbnail with the percent label once you are in zoom mode. I guess those of us that have been using Aperture since day one found that it is intuitively the "Z" key to go back to "normal" where Aperture determines how to fit the image in the viewer on it's own. For old time Aperture users it's intuitive because that's all there was - the Z key - to toggle 100% and normal. For those new to Aperture and happen to be using some other method of activating zoom mode on images, like the command + and - keys, I guess it may not be apparent. Once you use an alternative method to zoom the image you can't just back it off to go back to normal as one might expect.
The answer is no matter how you got into zoom mode - with a gesture, a menu choice, or the command keys - just hit the Z key to get out. Simple as that.