Hm… I have *a lot* of plugins, but I didn’t notice this happening until after the end of January or so. I guess I could narrow it down to what plugins updates have occurred since then (come to think of it, there’s been a WP update — 2.7.1 — since then…)
Meanwhile, I’ll make .htaccess unwritable.
If anyone else has experienced this, and narrows it down to a particular plugin, please speak up!