Huh, interesting question. I would imagine you are certainly paying for google ads, but it would be nice to hear from Apple on the iAd point.
After using an iAd supported game that has two iAd banners (slot machine game) on 3G for about 10 minutes, I had a very minute increase in data usage. It was the only app open, all others were closed and I disabled auto-download of e-mail and etc.
I checked via settings, since there is a delay with the AT&T app. I don't remember the usage but it was minimal.
In essence, my conclusion was yes, you are charged for it, in the sense that it uses your data your paying for, but it is so minimal I think it's a non-issue. I too, would like to hear Apples comment on it, but it's nothing I would worry about, personally.
I'm going to re-do my little "trial" here and post exact numbers (previous I did it out of my own curiosity and didn't record the usage)
-John
Edit: Using an app called "dataman lite", and closing all other apps, and resetting it to zero, I got 382.2KB of usage after opening, spinning once, and subsequently closing the slots app. This app opens two banner ads, but doesn't appear to ever change them, I didn't tap any ads. That tells me that a banner uses 200KB-ish of data, which is actually more than I thought, which means roughly 5,000 ads per GB, or that each ad is 0.01% of your monthly allotment (assuming the 2GB plan, with the 200MB plan it's 0.1%).
Still not entirely significant, in my opinion, but all users combined it probably adds up to a significant amount of data. I doubt any of it would change.