Economic cycles are a fact of life in the advertising business. It’s been true since the beginning. But perhaps more predictable than advertising cycles are advertising forecasts. Hey, there goes another one!The latest significant advertising forecast is from media agency ZenithOptimedia. According to Ad Age’s coverage of the report, marketers will spend $195 billion on North American advertising next year, 4.1% more than in 2007, while ad spending worldwide will near $486 billion in 2008 for a 6.7% gain. However, U.S. ad-spending growth this year will reach just 2.5%, far below the 3.7% growth it forecasted last summer and well under 3.6% inflation so far this year. That means that the respective growth of the entire pie is happening elsewhere, particularly inless developed advertising economies.But that 6.7% growth for 2008 is not so significant when you consider three of advertising’s most important catalysts are colliding that year: the U.S. presidential election, the Olympics and a European soccer championship. Once that’s over, you can bet all eyes will be on early 2009, seen in many industry circles as a likely peak for the inevitable advertising cycle to begin its downward trend.