[cmelbye]

Do they really? It seems more likely that they'd simply use a wildcard.

[tmx]

That'll teach me to post late at night. I forgot wildcards existed.

I was thinking that (if I built tumblr) they would do subdomain searching at the DNS level to avoid hitting their database.

Makes it even worse if they are using wildcards.

[X-Istence]

But if you don't publish an A record you get an ugly browser error message, whereas publishing the wildcard gives the user a notice saying that no such blog exists.