Online ad spending data from the Interactive Advertising Bureau (IAB) and PricewaterhouseCoopers (PwC) for the first half of 2008 is in.

The numbers seem generally strong, showing double-digit growth compared with the first half of 2007 in several categories: search, display—which includes banners, rich media and video—and e-mail ad spending. And the total US online ad growth rate of 15.2% is nearly the same as eMarketer’s 17.4% projection for all of 2008.

Yet online classified ad spending was down by more than 5% and may turn out to be a canary in the coal mine, showing the first signs of dizziness in an increasingly toxic environment.