Interesting post on MediaPost regarding the London OPA. Essentially online advertising spending is concentrating more heavily on the big four (AOL, Google, Yahoo, and MSN).
Does this mean the end of for smaller sites? My guess is that it means third party networks, and other companies that help leverage network effects of data from other sites will become more important. Behavioral and relational targeting will be critical for smaller publishers to maximize their yield through the attraction of media buyers.
If I'm an agency looking for unduplicated reach, it's hard to do that by making many small buys. If I can buy those through a network I can implement frequency capping easily across all of them. My guess is that's why you're seeing more money flow to the big four, it's the scale and efficiency as well as game changing custom offers that also offer reach ~ AOL's Gold Rush for example.