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!