I'm not an iPhone developer either, but I can't find anything in the docs that would seem to allow you to set a sampling rate. What you do is tell the OS that you want location data, and give it a desired accuracy (AccuracyBestForNavigation, AccuracyBest, AccuracyNearestTenMeters, AccuracyHundredMeters, AccuracyKilometer, AccuracyThreeKilometers) and the OS decides when to give you updates. It could be that Motion-X isn't using an appropriate resolution, but I'm somewhat dubious that that's the case, given its intended purpose would need sampling at more than a 30-foot resolution.
Relevant docs: http://developer.apple.com/iphone/library/documentation/User...