If you turn on the Swipe Down with Three Fingers gesture for App Exposé, you can quickly launch App Exposé for apps in the Dock.
Put the mouse cursor over the Dock icon of the app you want to run Exposé on, and then swipe down with three fingers. This will launch App Exposé for the app, even if it isn’t currently running! In that case, it will show a row of recently opened documents at the bottom of the screen, which you can click on to launch the app and open the document.
[crarko adds: It works, and it's kind of freaky in a cool way. For some reason, I couldn't make it work with the four finger swipe for App Exposé, but that may just be my fingers.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110806052156934