I finally got around to setting up a duplicate wordpress install to test this issue. Turns out it wasn’t the upgrade to 2.7.1 that caused the issue, as it exists under 2.7 as well. I shouldn’t have been so hasty in jumping to that conclusion.
Going through my plugins, I’ve discovered that wp-typogrify is the culprit of all the above issues. Disabling it brings back the proper wordpress alignment & caption behaviour. I see on the author’s homepage that he is merging his work into the wp-hyphenate plugin. I’ll try using that instead and see how I fare. Hopefully this info helps resolve others’ issues.
[Edit: I incorrectly stated the author of wp-typogrify is ceasing development. That’s not necessarily the case, he’s just in the process of merging with wp-hyphenate.]