A new forecast from ZenithOptimedia predicts that the ad spending will jump 6.7% globally to $486 billion. For online ad spending, the news is especially good. Zenith expects Internet ad sales to hit $44.6 billion in 2008, and increase its total share of the market to 9.4% (up from 8.1% this year). Zenith predicts that that Internet will overtake radio in percentage of total ad spend next year, and overtake magazines in 2010.

For other forms of media, the outlook is less rosy. Newspapers, magazines and radio are all expected to lose share, while outdoor ads, cinema advertising, and television will more or less stand firm with very modest increases predicted for each. TV will still be the dominant form of advertising in terms of percentage of total ad spend.

But even television faces and uncertain future, where the writers’ strike could be very damaging to the industry as a whole if it drags. “In 1988, it lasted 22 weeks, which is clearly a long time,” Zenith CEO Tim Jones told AdAge. “The three major networks lost 6% of their audience to cable. And that was never recaptured.” Could the writers’ strike bolster the Internet this time around?

The best news for the industry as a whole might be that it is a so-called “quadrennial kick” year, in which major events like the Olympics, Euro 2008 soccer tournament, and US presidential elections coincide. Those events are catalysts for big ad buys and could help drive growth in the industry in 2008.