Cisco has recently updated their forecast of mobile data growth. The good news is their forecast compound annual growth rate (CAGR) is down, from last year's forecast of 131% to their current forecast of 108% growth per year.
Unfortunately, when you look over the past 15 years of Internet traffic growth, the idea that any specific sector could exceed 100% growth for more than 1-2 years just isn't credible.
In the early days of the Internet (prior to 1994) traffic did approximately double each year. And, as Andrew Odlyzko points out in his classic study, there was a period of perhaps 18 months in 1995-1996 when US traffic grew much more rapidly, however growth reverted to 100% per year in 1997. And since then, Internet traffic growth has been slowing.
The most comprehensive data I am aware of is from Minnesota Internet Traffic Studies (MINTS) which shows the rate of Internet backbone traffic growth slowing so that in recent years (2008-2009) it has been in the range 40%-50%. For a good discussion of long term trends see Internet Growth Trends & Moore's Law.
Mobile network traffic growth
It's interesting to look back on the growth of mobile networks so far. Here are some numbers on cell sites, subscribers and voice minutes of use, extracted from CTIA data by Andrew Odlyzko.