It is the up and down arrow keys in ACR (and PS). (At least, it is on the Mac; dunno about Windows.)
Shift-up/down works in larger increments. The tab key moves to the next parameter, and shift-tab moves backwards...
That's good to know. I never realized that.
I just checked and Lightroom is different (at least LR3 Beta2): you use the + and - keys to move the current slider up and down (and if you try the arrow keys, a reminder pops up briefly in the middle of the image you are editing.