I'm certain that many companies would be willing to setup a custom solution, but you'll want to use the Host Quote function of WHT for getting bids on that.
One thing you may want to look into, is an appliance from BlueCat Networks (http://www.bluecatnetworks.com/produ...nis/index.html). From what I understand, it will handle DNS management on your behalf. Simply purchase one and co-locate it (or see if a host will buy one for you and lease it to you).
Anyone that needs dns services on that level are gonna more likely set up their own separate server in three separate rack spaces for it in their own co hq's in various locs around the world and save big time$$$.
Besides, someone that needs that level of service would never post that question in the following fashion:
Are there services out there that provide dedicated (or some other) DNS servers? (For those who'd rather not maintain 2-3 machines just for DNS.)