Since the new version of the iPhone’s operating system has been released, at least half a dozen software vendors have announced turn-by-turn, GPS navigation systems for Apple’s mobile phone. However, when a company that’s already sold 30 million standalone nav systems decides to join the fray, it’s a sign this app category is starting to be really serious business. Last week, after two months of anticipation, Amsterdam-based TomTom unwrapped its eponymous app for the iPhone and started selling it on the iTunes store for $99.99.