If you're downloading the content through the cell network instead of WiFi, probably. But, this would be no different than any other ad-based app, including the ones that already exist and serve ads now. Nor would it be any different from viewing an ad embedded on a web page.
In other words: ads aren't new. iAds however, probably make them more streamlined and hopefully use less data than the current alternatives. You're probably looking at a couple kilobytes, tops.