In December 2011 12% of paid search clicks in the UK came from mobile devices

"During December 2011, 12 percent of all paid search clicks in the UK were made either on a tablet or a smartphone, with 88% of all clicks originating from a desktop computer. This represents a 49% increase in mobile click share since October.
Share of search spend through December 2011 on smartphones and tablets represented a total of 7.5 percent whilst desktops clicks accounted for 92.5 percent of paid search spend.
The disparity between mobile click share (12 percent) and mobile spend share (7.5 percent) highlights favourable performance conditions for mobile advertisers. Relative to desktop search, volumes are growing faster than budgets, resulting in less expensive clicks on mobile campaigns.
Across all devices, paid search click volumes increased 43 percent, click-through rates (CTR) increased 24 percent and cost per click (CPC) decreased 5 percent during Q4 2011 on a year over year basis. The combination of improving CTRs and declining CPCs, point to significant efficiency gains for advertisers over the past year."

Source:  Data from Marin Software, reported in a press release, 19th January 2012