Pingback is back
Fixed. The problem was in the file matching expression in my .htaccess file. When I first converted this site to use Cheetah templates, I was quite pleased that I could simply put the http headers in the same file. Even my 404 template is done this way.
When I started caching, I found out that this didn't work out so well. I had to redundantly code this same information in my htaccess file. Bummer. I looked into mod_asis, but it was a little too asis... I want things like last modified and ETag headers...