When you say "an editing session" are you referring to working on a single image? Or groups of images where you close the ACR dialog and re-open it several times? Are you using local corrections?
I don't remember specifcally if the problem arose during local correction use w/ CS4, but I know the problem happened w/ CS3. I do remember that it happened in CS4 when I was simply moving exposure, WB, ect. sliders around (I might have already applied local corrections- don't know). Also, there may have been a few additional pictures in the sidebar, but I was only working on one at a time.
What gets me is that the problem has happened after several (15-30) minutes of use, during which time all of the controls were snappy and crisp. It just seems like something is degenerating, but I have no idea what. A complete reboot would cure the problem, although I don't think a simple restart of ACR made any difference- that's why I'm leaning toward some kind of RAM leak. How do I check for that?