I figured this out, BTW. The problem is that sonsofsamhorn.com (which somehow ended up in my browser history on my new machine) is passed along to PHP rather than doing a redirect. Obviously sonsofsamhorn.net cookies don't work on .com, so the front page always looks logged-out.
If you tell nginx to do a 301 from all .com URLs to .net (rather than just blindly passing it), that'll fix that error case.