I have been using one of these for my iPhone 4 for a year or so. Works good. My only complaint is a slight rattle where the case mounts to the adaptor (a bayonet style mount). I shimmed the bayonet with some tape and it helps. They make mounts for most of the smartphones including the iPhone 5.
http://www.thinkbiologic.com/product...mount-iphone-4
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2010 Cannondale CAAD9-4, 1978 Jack Taylor "Tour of Britain" #7338, 1969 Jack Taylor Ladies "Tourist" #5935 (wife's), 2010 Specialized Dolce (wife's)