I’ve managed to enable comments on Tumblr by using Disqus.  It’s a pretty neat service that aggregates comments from other sites using Disqus.  You can comment via Twitter or Facebook!

I initially had a configuration problem:

“This page could not be reached by Disqus.  If you are testing locally or on a password-protected page…”

As I’m using a custom URL (http://tumblr.dilly.ca), I figured this may be the problem.  I managed to fix it by changing my Site configuration in Disqus to use the Disqus URL (http://dilly.tumblr.com).  Seems to be working fine now!