An FT.com article reports on findings from O2's recent launch of the iPhone in the UK.
One interesting statistic that was revealed was that 60% of U.K. iPhone users are sending or receiving more than 25 MB of data a month. This information alone is difficult to interpret, but O2's chief appears to believe that this represents a large increase in mobile data usage.
They try to compare it to the usage amongst non-iPhone O2 users (only 1.8% of other O2 customers use > 25MB/mo), but this is unhelpful as the iPhone would tend to draw a more data-centric audience. And many phones have no data-options available to them, but would also be counted in that percentage.