Each new blog post was causing all the previous posts to appear as new in Google Reader and in Vienna. Looking into it, I found that the pubDate elements were all just ":00", because loathsxome wasn't setting the variables the RSS template uses. So I added them to my customized fulldate plugin.

Notably, ams's blog doesn't have this problem. I assume he's using the default template, which doesn't include pubDate at all, and thus sidesteps the problem - so maybe I should just rip it out.