This not exactly legit app, which its makers, iPhone Savior, named the Star Radio Communicator (so it's safe to assume this is not an official Paramount-licensed Star Trek product), gives your iPhone the look of the classic communicator from the original '60s TV series.
According to
Gizmodo, "the 99 cent app gives you a dialing interface to place calls from within the app, making you feel like you're about five seconds away from bitch-slapping a couple of Klingons."
Yet another reason for those that held out on buying an iPhone (like me) to get the 3G S.