According to data from a recent Forrester survey of roughly 20,0000 Europeans (reported in Total Telcom), on average 13% of Western European consumers now access the Internet on their mobile devices, while and 50% know their phones are Internet capable. 

The numbers in the chart below vary by country and the data tend to change when you ask users about daily or monthly frequency, but in the US/Canada, the percentage of mobile Internet users is north of 20% (depends on the survey). Our figure is about 27% - 29% for the North American market (excluding Mexico).

There are two very clear variables that drive mobile Internet usage: the price of data and smartphone adoption. As data prices (and handset prices) come down more people will opt for smartphones and unlimited data plans. This is very analogous to the early days of the Internet on the PC where dial up gave way to unlimited broadband and usage grew dramatically. 

Mobile is a more accelerated market. But things that may further speed up adoption, like carrier price wars, cannot be easily predicted. Overall, the cost of data and WiFi access should still come down over time, although smartphone handset prices will likely remain between $99 and $199 in the US. 

Most mobile Internet ad revenue forecasts are tied to smartphone growth and penetration as a proxy for Internet access and usage. In general, I would tend to agree, but there's more nuance within the category because all smartphone owners don't equally access the Internet: iPhone and Android owners do more than BlackBerry or Windows Mobile owners for example.