In the online utopia, there was supposed to be ad inventory as far as the eye could see - into infinity and beyond, theoretically - but "theory just ain't what it used to be," writes Steve Smith in OMMA Magazine. Admittedly, the crunch is in specific categories, such as auto, pharmaceuticals and entertainment. "Jumpstart's network of contextual auto sites was 100 percent sold out in 2005," says Mitch Lowe, the CEO of Jumpstart Media.

Obtaining prime spots on, say, TVGuide.com and Yahoo Entertainment and Movies can create scheduling conflicts as movie studios attempt to run ads on similar days and day parts. Though there's unsold inventory, it's not where advertisers want their ads.

Pharmaceutical clients, for example, "want condition-specific content in very targeted verticals, and they are willing to pay a premium if you can supply them," says Paul DeBraccio, CEO of Interevco, an ad rep firm.

"So will there be a[n]… upfront for web ad sales? It already exists on an ad hoc basis," writes Smith, citing a couple of examples.