Somehow I managed to toggle the mode where Alt-LMB moves a new object toward and away from camera on the cplane instead of up and down on it. A new, blank hipfile doesn't have this problem, but the one I've done a lot of work in does. I can't find mention of this in the help anywhere and searching for ‘h9’ on these forums doesn't work because ‘h9’ is too little information for the search engine.
Thanks in advance,
-john