Semiologic opt-in front page plugin stops posts appearing in RSS feeds

that’s what it’s supposed to do: the main rss feed is that of your front page.

use category_url/feed/ to get the feed for other categories.

So would it be the case that the only way to get a feed containing all articles would be to have an ‘all articles’ category into which every article is placed, and then direct people to that feed?

The Semiologic ‘subscribe me’ plugin that I am also using sends people to site-root/feed/ wherever they are on the site. I’m going to have to modify one or the other, aren’t I?

yes to both questions.

your /feed/ reflects what appears on your front page. I used to get frequent requests along the lines of ‘/feed/ has every post in it rather than only those that appear on the front page’. and this is indeed the expected behavior by most uses.