Political scientist Eliot Cohen has a thought-provoking WSJ op-ed to the effect that government officials generally don't pay much attention to outside commentators; serving recently as an aide to Condoleezza Rice, he found he was focused on info from inside the government, and punditry was basically background noise.
Historically, though, there have also been times when pundits seem to have influenced government, as with Commentary in the late 1970s paving the way for much Reagan foreign policy, and supply siders at the WSJ doing the same for tax policy. It also may be that Eliot's thesis is particularly true in foreign policy, dependent as it is on confidential communications. Economic policy in recent months was evidently influenced, at least for a while, by academic economists saying put government money into the banks, rather than buy up their bad assets.
In the film Arguing the World, Irving Kristol said that a journal with a circulation of 100 people could change the world. As for the impact of little-read blogs, that remains to be seen.