Searches from iOS devices account for 60% of mobile searches in the US

"Barclays Capital’s Anthony DiClemente has an interesting theory about Siri and its effect on Google’s mobile search revenue. DiClemente points out that iOS grabs a whopping 60 percent of mobile traffic share according to Comscore’s December data. When you look at Google’s mobile search statistics, the bulk of the traffic is likely from customers using iOS devices to search using Google.
As Siri takes a bigger role in iOS, customers will increasingly use voice-assisted search instead of mobile search. Siri doesn’t use just Google for search; it can pull information from Bing, Wolfram Alpha and other mobile services. This shift from Google search to Siri search could have an impact on Google says DiClemente . Google estimates that mobile is about 20% of all paid search items and iOS is likely the biggest contributor. Any loss of customers to Siri could, in the long run, hurt Google claims DiClemente."

Source:  IntoMobile, 10th March 2012

Note - I'm assuming that this is a US-only stat - let me know if you know different!