For some as-yet-unknown reason, anonymous comments don’t seem to be working on my installation of Drupal 6. Which is a bit of a pain in the arse.
I’ll try to figure it out, but for now, if you want to comment on a post, you’ll have to register (which is quite safe - I don’t use the email addresses or do anything else nasty with your data).
I’ve worked out what was going on - somehow my anonymous user had got removed during my upgrade and server migration.
These Drupal threads helped:
The solution was just hack the users table manually and add back a user with uid = 0. Scary!