There's not a lot of details on this at the moment, but the Financial Times is reporting that US antitrust regulators plan to investigate Apple's mobile advertising practices to see whether they unfairly restrict rivals like Google and Microsoft. As you'll recall, Apple recently revised its rules on outside advertisers and, specifically, their ability to collect analytics on ads for the iPhone, iPad and iPod touch, effectively cutting out Google's AdMob in the process -- something the company's CEO has publicly complained about. What's next? That's still not certain -- the Financial Times says that while US regulators have taken an interest, it's still not clear whether the FTC or the Department of Justice will handle the investigation going forward.