I read about the new Shortcut SUMO today on Scott Kelby's blog today and picked up a copy of it. It's pretty cool. There's a section for Adobe Camera Raw Keyboard shortcuts that's free with the basic application. Then there's another section that covers Keyboard shortcuts for Photoshop CS6 that's $2.99 and sections coming soon for Lightroom, InDesign and Illustrator (presumably for $2.99 each). These new sections will be in-app purchases when they are available.
The interface is pretty smooth. There's a page for each keyboard short cut and you can access the short cuts by scrolling through menus.
Of course the idea is that you can have your iPad sitting along side your keyboard while you are working and quickly find the shortcuts you are looking for. As Scott says in his post, this really is more like a ebook than an app. You get a chance to choose if you are a Mac or PC user when you first start the app. However, when you search by keyword it displays both Mac and PC shortcuts. Scott promises to fix this in the future.