Aside from already being a great bargain, there's is a side benefit to having Adobe Photoshop Elements 9 on the Mac App Store. Like all Apps from the Mac App Store, you'll now be able to install Photoshop Elements on as many Macs as you own for the one price. That may not mean much for some, but for those that found the license limit of two computers a bit too limiting, you'll now be able to install Photoshop Elements on each of your Macs. Of course this only applies to newly purchased copies via the Mac App Store and not existing shrink wrap licenses.
You can get your copy of Photoshop Elements 9 for $79.99 here from the 
Already own Elements?
Here are several free videos on how to use Adobe Photoshop Elements.