Take a look at Chris Gaither's LA Times now widely circulated piece about how the Web is stealing advertising from TV. Competition from online and cable, declining audiences, rising prices and a lack of accountability are a kind of perfect storm of factors putting pressure on broadcast TV ad rates if not advertising in the medium itself.
But this is just one window into what's happening across the board with all traditional media to varying degrees (radio, print Yellow Pages, print newspapers, magazines) vs. the Internet.
Online offers both measurability and targeting — and it's a lot cheaper right now, especially vs. TV. This is also where desirable audiences are going. The problem, as has been repeatedly discussed here and elsewhere, is that the inventory hasn't yet been developed that would allow the kind of revenues online that exist in traditional media.
Because of the nature of the medium, the online audience and its behavior are fundamentally different from TV or radio audiences, for example, where large groups are exposed to the same messages at the same point in time. Building out scale comparable to traditional media involves development or extension of the network and simultaneous development of the tail.
And, I believe, the tail is largely (though not exclusively) about local and to a lesser degree SMEs.