Ken Burns in iPhoto

You can only adjust the direction of the Ken Burns effect, but not the amount.

After selecting a slide in the slideshow, open the settings and click the "This slide" tab. There you can enable or disable the Ken Burns effect for each individual slide and select if you want to zoom in or out.

You can zoom in and move the photo so the start point is anywhere you'd like it to be:

Then move the slider to End and zoom in/out to where you want the end point to be:

And you'll get something like this:

This is with iPhoto 9.6.1.