According to the latest State of the Mobile Web report from Opera, Google enjoys mobile search dominance in numbers generally consistent with its lead and share on the PC Internet. Yahoo!, however, is stronger in mobile than it is on the PC. Bing has a tiny presence by comparison.
One thing to keep in mind, however, is that these data are drawn from Opera Mini users and don't capture the iPhone and most activity on the other smartphone platforms. However I can tell you that directionally it's the same on most smartphone platforms.
Below are the customary data from Opera on top sites and handsets. The search share data are at the bottom. The full report is available here.
Opera has developed a version of its mobile browser for the iPhone and will be submitting it for approval shortly. It was demonstrated at 3GSM in Barcelona and received rave reviews. Apple will have a potential anti-trust issue in the US and Europe on its hands if it declines to allow third party browsers on the device.