My startup NameTerrific can support instantaneous DNS updates in a geo-redundant Anycast infrastructure. As long as your TTL is sufficiently low (<300), the impact is quite limited as propagation time is negligible at NameTerrific.

EDIT: Sorry guys. We got some issues with a gem after installing the recently updated ruby2.0.0p0. The unicorn workers were timing out. TerrificDNS is completely unaffected and the site is already running again.