Since the iPhone launched the default search provider for the handset has been Google. That means when an iPhone user opens the Safari browser any searches are conducted via Google. Apps are available to let you choose other search providers already.
Microsoft has a Bing app for the iPhone for instance. According to reports, Microsoft and Apple are in talks to swap Google for Bing as the default search engine. Speculation has the reason being that Apple and Google are battling each other in key markets.
If the negotiations work out for Microsoft, it could possibly become the default search engine for Safari on Mac computers as well. That would be a good thing for Bing as it tries to overtake Yahoo and Google.