Woohoo! Finally found out what the problem was with BlogEngine comments. It turns out the problem was an element in the parent web.config called xhtmlConformance. Here's an article about it in Scott Guthrie's blog:
Gotcha: Don't use <xhtmlConformance mode="Legacy"/> with ASP.NET AJAX
I had figured it wasn't really a problem with BlogEngine, and was probably some sort of configuration issue, for this simple reason: the site was identical (except for content differences) on my server, vs. my local machine. In my case, the legacy switch had been turned on in the web.config of the parent web... which is why I had trouble tracking this down. Not knowing about the legacy switch didn't help the situation.
Thank you Scott Guthrie!