This occurred retroactively on a post that was displaying properly before installing and activating the plugin. I'm running a fairly minimal plugin set right now, and I can't see any other active plugins that might change content. At the moment, I have a Twitter auto-poster, Akismet, Feedburner Email Widget, and a plugin that adds "pending review" posts to the dashboard.
I was able to use Chrome's inspect feature to deduce that paragraphs had been collapsed into a single paragraph. Once the plugin was disabled, the paragraph spacing was back and, according to Chrome's inspector, the paragraph tags were back as well.
I have since deleted the plugin from our site, but I should be able to reinstall it on our test site to see if the behavior is the same. Once I've had an opportunity, I will come back here and post a link so that you may take a look. Thanks for the help!