Well I already posted a link to zoneedit, which offers a fairly cheap service for it. I'm not entirely sure if you need to use them as a registrar in order to take advantage of said failover services, but it doesn't sound like it as, and I'll quote their faq:
You can setup the IP of another server (from a different hosting provider) to take over in case your main site fails.
You'll need to keep in mind a few things however if you plan to use this service. For one, it will not transfer over any modified files or database changes. If your (or your client's) sites are all static (ie. html) and don't change frequently, this service works perfectly. If not, you'll find yourself looking to use another hosting provider that offers shell (*unix) based access, as you'll want to rsync all of your (or your client's) data regularly. For the database portion, if you had one, this would complicate things to even further extreme measures, which I won't delve into do to the complexity of such a setup.
Personally, I believe zoneedit's failover services would be great to provide a custom "Hi, we're down right now but we'll be back soon" -ish error page in case DH does happen to go down. Beyond that, the setup becomes far too complicated.
Chips N Cheese - Custom PHP installs and the like!