Forgive me for disregarding CS4 in this tip, I went from CS3 to CS5 so I have no idea what was or wasn't changed in CS4.
In CS3 and back you could easily select a layer by directly clicking on it's contents on the canvas, which made it easy to find and select a layer and quickly move things around. In CS5 I can no longer simple click on something. I spent forever and a day googling this without any luck. I tried a bunch of different keyboard combinations and still nothing worked. And like most great discoveries, this one came to me accidentally while I was randomly smashing keys and clicking things.