The market for mobile display advertising is still small, according to eMarketer. As of March last year their estimate was that the market size for 2008 would be $142m and for this year $338m.
It is a safe bet that between March last year and now those forecasts have come down.
This tells me that it will be very hard to build an ad-funded pure mobile play, even when the market picks up again.
I’ve always thought that for the majority of services an integrated web and mobile approach makes the most sense. This data adds weight to that thesis.