Google helped loosened Apple’s tight control over the iPhone a little today by launching Google Voice as a Web app for both the iPhone and Palm OS.

Google Voice provides a single phone number for multiple phones, provides low-cost international calls, transcribed voicemail and a number of other features.

The move is significant because last summer Apple controversially removed Google Voice from the App Store, saying that it duplicated features already available on the iPhone. This prompted an FCC inquiry, and Google promised at the time to find ways to bring its services to users “one way or another.”

To get the rich features that come with Google Voice, the company made use of HTML 5, a new Web technology being built into browsers that lets them run more sophisticated features without the need for plugins. This is important because the iPhone doesn’t support Adobe Flash, which can be used to built rich internet applications.

The approach is brilliant and potentially deadly–there’s no way to stop a Web app without turning off the Internet. Perhaps it could be the beginning of a backlash against the App Store. As mobile apps get better, developers may lose patience with the arcane, and sometimes seemingly arbitrary, process of Apple’s app review.