Users of Google's instant messaging service now can send unlimited free text messages on their iPhones thanks to a new application from the folks at Seattle's Innerfence.

The 99 cent application, dubbed Infinite SMS, has has quickly risen into the top 100 paid applications in Apple's app store. At the moment, it is also the second most downloaded paid social networking app, with the company selling more than 1,000 copies in the first 48 hours. Innerfence co-founder Derek Del Conte said the Infinite SMS concept is not new, but the design and implementation is.

"There are a lot of way to send free SMS messages, but existing apps don't expose their free messages in a way that feels like SMS to users," explained Del Conte. "Most of them are send-only, and they tend to expose the technical details of the implementation, and because of that feel more like an instant messaging client or an email program."

By using the Google instant messaging service on the back-end, Del Conte said that they can deliver messages on behalf of users in a way that looks just like a text message. iPhone users pay extra to send text messages, either 20 cents per message or monthly plans that range from $5 (200 messages) to $20 (unlimited).

Obviously, cost-conscious consumers may like the idea of paying a one-time fee for the unlimited access. But wouldn't Apple or its carrier partner AT&T frown on this sort of activity? Here's what Del Conte had to say about that.

"It's just a matter of degree: we support 2-way messaging and we've taken pains to feel like SMS," he said. "But since SMS-sending apps are a well-established class within the app store, I would be very surprised to see Infinite SMS treated any differently by Apple."

Infinite SMS has some bugs and hurdles, such as the inability to send messages to small carriers like Cricket Wireless and MetroPCS. There also appear to be some limits on the number of messages sent within a short period of time and people must activate the service with the recipient cell phone number first.

Infinite SMS marks the fourth iPhone application for Innerfence, whose $49.99 credit card terminal application I wrote about last year.