Check out the whitepaper, it has a lot of details about that. :)
Direct link: http://pages.optimizely.com/CDNBalancingWhitepaper_GeneralLP...
Direct link: http://pages.optimizely.com/CDNBalancingWhitepaper_GeneralLP...
My theory is that they're using the Dynect CDN manager. This is a tool created by Dyn to let companies use their DNS system to pick which CDNs are used where, and to create round robin rules that are percentage and region based.
I confirmed this by checking their nameservers, which are currently pointed at Dyn (ie, NS1.P24.DYNECT.NET).
The thing that amuses me about this is that Edgecast offers this same service at a far, far more reasonable price. I'm not sure why they didn't just use them.
""" At the highest level, a “balanced” CDN architecture is one that leverages two or more CDNs hosting identical content to increase (a) the overall number of physical Points of Presence for the network (PoPs) and (b) the proximity of those PoPs to the end users accessing them around the world """