I upgraded WP and Atahualpa and, post-upgrade, only the excerpt is showing when viewing the actual page. The home page, too, shows only the excerpt even when it is set to show the full post. It's clear that what is showing is the excerpt, since I can change its length by changing the excerpt length setting, and I can control what shows (images, links, etc.) by changing the "don't strip tags" setting.
Enabling a different theme fixes the problem - the posts display normally. The posts look normal when edited. Weirdly, even doing "preview" of the normal-looking post displays the excerpt. Thoughts?