Hmm. An interesting observation: apparently, the mysterious background disk write activity on my wife's EeePC has now ceased. This was the case that had caused my initial concern, and was therefore the reason for this thread. Some months ago, the original Minime 08 install on her 701 suddenly ceased functioning; investigation revealed the problem was caused by the failure of its SDHC card, upon which the /tmp, /var/, and /home directories had been placed. I had then replaced the card, updated the machine's BIOS, and re-installed with Minime 09 - then discovered the background disk write problem.
I was quite concerned, yet didn't know how to rectify the mysterious behaviour - thus prompting me to begin this thread. I subsequently confirmed the behaviour in other installs since, and have not found any solution - yet now, on that one machine at least, the problem seems to have somehow resolved itself? How could that be possible? Her usage pattern is very minimal; she does use her EeePC often, although basically only for browsing her two favourite websites: YouTube and FaceBook.
Meanwhile, the background disk write behaviour yet continues on my other installs - such as the 2010 b2 install on my recently-acquired Acer TravelMate 2200, which is now serving as my 'main' computer these days. One additional difference was noted, though: with the Minime 09 install on my wife's EeePC, the background disk write behaviour was apparent immediately - whereas with the 2010 b2 install on my TravelMate, it was not initially apparent and instead began to develop only after several days of usage.
Another possibly pertinent observation is that the behaviour also seems to be dependent upon the selection of desktop environment. With the TravelMate setup I had also installed some alternative windowing managers - and while the disk write behaviour does occur with KDE, I have noticed that the same does not occur when I boot the machine into the Enlightenment environment. I shall repeat the test with some other window managers now, and report the results presently.