A useful summary from the Publishing 2.0 site:

Journalists and PR professionals, the influence brokers of traditional media, have lost a huge degree of influence on the web in large part because they don’t link to anything. While traditional media brands are still powerful channels on the web, they are losing influence everyday to the link-driven web network — journalists and PR professionals can no longer depend on controlling these former monopoly channels to exert influence online.

What makes you influential? Having a big network that’s interested in the same things you are. Not rocket science, really