Actually, there are a few major GTM (Global Traffic Management) schemes that do use the IP address of your DNS server, rather than your actual IP. They basically abuse the DNS system with super-short TTLs and give a different response to the DNS query based on the IP of the downstream DNS server. So, if you use a DNS server located on the east coast of the US when you're on the west coast, you'll get an east coast server even if that service has a west coast datacenter available.
This is done primarily to free companies from the burden of having to design proper geolocation into their app/service, turning it into a more plug-n-play solution while breaking several of the finer points of DNS (like proper caching). This type of traffic management could easily be contributing to Comcast's DNS troubles, as it drastically increases load on the entire DNS infrastructure. Paul Vixie did a good detailed write-up about this type of traffic management a few years back. Unfortunately it's probably here to stay, and is used by some very major corporations and online services.
If you want the most reliable DNS service, and want to be directed to the closest servers for the services you use, your only real option is to run your own recursive name server. A simple caching name server isn't enough, and will curse you with many of the same problems you see from your upstream. Fortunately, recursive name servers are pretty simple to set up, in both the *nix and Windows worlds.