Nope, it was just these 3 (as we were deciding between DNSMadeEasy and Dynect, and wanted to throw Godaddy in there for comparison).
Although we currently use DNSMadeEasy, and due to the uptime issues we might be looking to switch at which point I'll cast a broader net. Of course if you have a pingdom account, setting up these DNS checks is quite easy. They have a special monitoring type just for DNS.
What uptime issues? We have used DNS Made Easy for over 8 years and have never seen any uptime problems at all. Checking one name server through Pingdom is not giving you a detailed review of name servers. You could have checked another name server in Dynect and they could of had twice as much downtime... but still Dynect was up.
Showing one name server is good, but it is not judging a providers "downtime". Would be much more useful if you tested all name servers. If several were down at once then yes... there is a good chance that you could loose a query.
