There are 3 subtle changes that have been made to the Keynote icon in iWork '09. I thought for the sake of, oh I don't know, fun? that I'd point them out here in a side-by-side comparison. If you find any other differences between these two icons, feel free to share them.
The entire icon, particularly the lectern's wood grain, has a slightly darker appearance.
The date on the speech notes has changed to "Q1 2009", obviously.
Although very subtle, the clarity of the paragraph text, has been increased.
The compression of the JPG graphic above doesn't make it very easy to see these subtle differences. But, you can easily see these differences yourself when you download and install iWork '09 since the new version installs into a new directory leaving the old applications intact.