From the June 7th Zone Files - Domains served by each nameserver... the Top 100:
Er, better to say "second level domains served by each nameserver". We strongly recommend our customers make use of third level domains under xo.com, which is one of our domains. We have an additional 1154 domains registered as third level domains for people who don't really "need" a vanity, second level domain. In our opinion, most people don't really need a vanity domain, and are quite well served with a third level domain. I'm even cocky enough to think we're helping preserve namespace this way. :)
I've always been curious how well bind runs with 10,000+ domains in it. That's one big named.boot file.
Use bind 8.1, and split up your named.conf file into multiple included sections. :)
-- Rusty Hodge <rusty@hodge.com>, San Francisco
Matt Petach