Better Way To Show Running Applications in OS X Lion
The easiest way to do this (in my mind) is to not have any applications permanently stored in your Dock.  In other words, the only applications down there are running ones. (This is what I do, and it works great; I can at-a-glance see what's running without having to use cmd-tab).

That being said, OS X is moving in the direction of "you shouldn't have to care".  Things like Sudden Termination and User Interface preservation are aiming to make it so that a process can disappear silently in the background, but quickly come back when you need it.