Russell Beattie thinks it is browsing (building on a Nokia survey):
The key takeaway for me was that browsing is the number one (by far) mobile application that uses data, and interestingly, carrier decks only account for 50% of the traffic. Thats great! That means people are actually going outside the gardens and hitting other sites. I think this trend is only going to continue as mobile browsers get better, screens get higher resolution, and network speeds get faster. Ive seen demos of Nokias upcoming Webkit based browser, and its very impressive. Lots of people have even used their mobile browser to access their email via a webmail account. Thats very cool – and points to the idea that people continue to fall into similar patterns. They *could* set up their phone to access email directly, but theyre most likely used to getting their mail via a web interface.
I personally think there is a massive vacuum right now in mobile-focused web sites. The numbers are clearly showing theres a growing trend for people to access sites remotely, sites need to really get up to speed in making their existing content more mobile friendly, and to do something innovative and compelling for those mobile browser users who may have different use cases for the information theyre trying to retrieve.