Google Talk is now available to iPhone, well, sort of. They've made a "Web Version" available for iPhone's Safari browser (by visiting: google.com/talk). This is essentially the same web-app found within Gmail on the desktop version; which means there will be no "VOIP" or talk functionality and if you are not using the active Safari window, it will assume you are signed off (unlike the much better BlackBerry version where it is a real application running on the background).
In other words, iPhone Google Talk is just about as useless as the OS X version. Nothing to look here, if you are looking for real VOIP/IM solution, may I suggest fring?