Most of the really heavy CDN users I know do have multiple CDNs constantly in operation, for contract negotiation reasons if nothing else.
DNS is interesting, especially if you do anything location-based. I currently use just CloudFlare, but am not convinced they have enough internal redundancy on DNS, so investigating using Dyn, or self hosting again, or Route53, or some combination. It gets a more complex if you want to use multiple providers doing anything beyond simple DNS though (the DNS protocol itself is totally fine for this, but most of the config management is provider-specific, and using normal dns zone transfers/notifies doesn't really work in this model)
DNS is interesting, especially if you do anything location-based. I currently use just CloudFlare, but am not convinced they have enough internal redundancy on DNS, so investigating using Dyn, or self hosting again, or Route53, or some combination. It gets a more complex if you want to use multiple providers doing anything beyond simple DNS though (the DNS protocol itself is totally fine for this, but most of the config management is provider-specific, and using normal dns zone transfers/notifies doesn't really work in this model)