Namecheap is a fabulous hosting provider. Love 'em. That said, highly-redundant DDOS-resistant DNS infrastructure is not their focus--they offer it as a free service with hosting, but it's best to cough up a few extra bucks and move your DNS to something more robust with AnyCast (DNS Made Easy, Route53...) when you can if uptime's important to you.
Wow - you're seriously a hero. Thanks so much for building the test and thanks (even more) for the blog post (especially the legwork on pricing). I look forward to the day when "can we jump on a call?" sales processes are well and truly dead and self-service and transparent pricing is the norm.
Check this out for real-time query speed testing: http://cloudharmony.com/dnstest
(And yeah, looks like GoDaddy is a solid "Test failed" still...)
Funny note after running the test a few times--the worst performing provider is the one whose salespeople contact me the most...