Since I posted the item about Google and mobile advertising yesterday, I've been giving the matter some more thought.
By coincidence, I had a pre-arranged briefing this morning with the CEO of a company (BuzzCity) doing mobile advertising, especially in emerging economies. They specialise in "Tier 2" users - essentially the emerging middle class in developing countries, plus some blue-collar workers in more mature markets. Interestingly, the base includes quite a lot of the elusive "mobile-only Internet users" I crossed swords with Tomi Ahonen about earlier this year. They have around 3m users in total.
The BuzzCity CEO came at my question of the realistic per-use revenues from a different angle - instead, taking a top-down view about the "share of total advertising" in a given country. He suggested that mobile might get to 5-10% of the total, on a 5-10 year view (this is in developing countries like Thailand or Indonesia).
I also tried to work out some metrics along the lines of my previous post. Based on some other numbers I got about the business model, I'm estimating that they're doing perhaps $0.20-$0.30 per active user per month, based on primarily WAP-site text and banner ads on low-mid-range phones. That's actually pretty good - and it could be argued that as the market matures, that should increase... although competition will probably push down prices, and I suspect they've got a lot of early-adopter advertisers still experimenting with pricing.
One thing that did come out though was that it appears that there are now a lot more cheap/easy prepay-friendly data plans coming out in some countries in the developing world. He cited examples of new tariffs (often per-time rather than per-MB) that massively increase mobile data access in some emerging markets.
Bottom line - mobile advertising is starting to grow quite healthily. But I'm still utterly unconvinced that Google's view of it overtaking fixed-Internet ads is at all probable.