Since when was running named on a pair of servers (put one on linode and one on slicehost for redundancy) not work for all but the larger companies? And by the time you are that large, cost of name servers is so insignificant as to be round off error.
I always used to think that I was missing something special about DNS - I mean, it can't be as simple as configuring named.conf for a master and slave, creating a zone file, tossing an SOA plus some Resource Records into it, could it? Perhaps it's because I keep a 500 page copy of "DNS and BIND" on my desk, and perhaps it's because large deployments of DNS _are_ complex - but for the vast majority (and certainly anybody simple enough to outsource there DNS to a hosting organization) - it's a pretty straightforward, solved and inexpensive problem.
The one reason to do this, I guess, is if you just are done with the "part-time-systems-administrator" gig, and would just like to wash your hands of it. But, if you have to do some of it already, the incremental cost of running for multiple organizations is pretty insignificant.
I realize, btw, that this is off topic - and you are actually looking for guidance on out-sourced DNS servers - I just couldn't restrain my fingers when I caught your paragraphs noting the cost of parts and servers. $40/month buys you two servers, one with slicehost and one with linode.
There are a lot of reasons to hand your DNS to a third party (Nice interfaces, handling the customer service, hassle-of-managing) - but cost of servers/internet isn't one of them.