Google (NASDAQ: GOOG) and Yahoo! (NASDAQ: YHOO) have been saying for years that the next frontier for internet-based advertising is the cellphone screen. With wireless handsets having high-speed data connections and really impressive (but small) screens, and with cellphones outselling PCs one would think both companies are right on the money. Not a quarterly conference call with Google can go by without CEO Eric Schmidt alluding to the mobile ad space as a whole new frontier for Google.

When a research firm like Gartner predicts $11 billion in industry revenue from mobile advertising in 2011, that is the kind of figure that makes many stand up and pay attention. But so far, mobile advertising reality is not turning out to be rosy as that. Mobile advertising is an industry worth under $1 billion in 2007; can it really skyrocket to over $11 billion in four years? Most likely, no.

A main reason for this is the incredible complexity of mobile web browsing on the mainstream cellphone (lack of a keyboard, perhaps?). I'm not talking Palm Treos, iPhones and other muscular, do-it-all phones, but the kind the average joe carries in that front pants pocket. Do you know of many people who regularly access the web on their cellphone? Think about it for a second. According to Jupiter Research, only 16% of Americans regularly access the web on all those cellphones. What's going to get that figure up to 50%?

For one, a much less convoluted way for normal phones to hop on the web, making it as easy as web browsing on a PC. It's nowhere near that now, save for a handful of device categories like smartphones and the like. Think text messaging could be the answer? Will customers stand for being interrupted with advertising over text messaging? It's an opportunity that could launch a consumer revolt more than anything. What's left, then? Mobile advertising will get there and indeed win the day -- just not by 2011 and in the numbers predicted.