It's not relevant how many people live in certain area because if these are "Super Pops", then each one will need to handle a large amount of connections, while giving decent latency to all corners of North America. It has to be a balance all the way around. Many times it depends on what ISP is being used and what backbone providers the ISP is using.
These cities would be fine.....
New York uses Chicago POP (5 hops 20ms) New York uses Dallas POP (8 hops 44ms) Washington uses Chicago POP (6 hops 24ms) Washington uses Dallas POP (8 hops 32ms) Miami uses Chicago (9 hops 32ms) Miami uses Dallas (9 hops 28ms) Atlanta uses Chicago (8 hops 24ms) Atlanta uses Dallas (8 hops 20ms)
Oh, and I do think you're spongeworthy -- Ian Barnes SofSwitch Communications Inc.