Thanks for that link, baltwo, but I probably should have clarified that I provide IT for a dozen Macs in a small office as well as freelance support, so I'm aware of, and have checked for, all the standard beachball causes twice over.
The only reason I even bothered posting was the cursor flickering symptom.
The unexplained beachballs (again, they occur randomly across apps, and do not correspond to disk access, RAM shortage, paging, or high CPU use), while more annoying to the users, are actually the secondary diagnostic symptom as far as I'm concerned; the cursor is much more specific and seems to point to SOMETHING messing with either mouse input or cursor positioning, despite there being nothing on either system that should be doing that.
I figured out what was causing the weird flickering cursor, and I'm now somewhat surprised I haven't seen more complaints about it: Adobe Acrobat/Adobe Reader.
I've tested on nearly a dozen different 10.6 systems at work, family, and clients, and every single one has displayed the same symptom when either Acrobat (8, 9, or X) or Reader (9, X, or XI) is running and active or there is a PDF being rendered by Adobe's Safari plug-in (as opposed to the Apple one). Opening an Adobe preferences window is guaranteed to trigger it, but it'll also pop up randomly.
I now realize the seeming randomness of it was due to the people seeing it having either PDFs open in Safari and my not realizing it, or Adobe Reader running.
The beachballs were a red herring; they may have been caused by the same thing on the two systems I was working on (both cleared up without me ever pinning down a definite cause), but they had nothing to do with the twitchy cursor.