This size of image is 16 kilobytes, the same size as one hundred full-length SMS messages of 160 characters each.

First a bit of background: About two and a half TRILLION SMS text messages are sent each year. In North America they are outrageously expensive. AT&T’s basic data plan for the iPhone costs $20.00 per month and includes only 200 text messages. Exceed 200 and you’re charged 5 cents for each additional message you send or receive. …and international texts get charged at 20 cents each. Unlike much of the world, US users get charged for receiving a text message as well as sending one. So a 160 character message that costs the sender 5 cents, and the addressee another nickel, is getting billed at an astounding $625 per megabyte. Spill over to 165 characters and it costs twice as much since it’s broken into two “messages”. No wonder the wireless carriers love texting!

Enter AOL’s new AIM app for the iPhone. The new 3.0 version of the iPhone OS includes a feature called “push notifications” so an app can “wake up” in response to a network event. The AIM app uses this to implement an instant messaging client that does an end run around the outrageous carrier fees. They move over either the WiFi or cellular data connection. And they do it as part of the flat $20/month unlimited data connection.

“But…”, I hear you say, “If the person I want to text isn’t an AIM user, I can’t text, right?”. Wrong… you can address an AIM instant message to a cell phone number and AOL will forward it via SMS. Your recipient gets charged by the carrier, but you don’t. Now if that isn’t a near perfect viral campaign tool to spread AIM as an SMS replacement, I don’t know what is.

All of this works on the iPod Touch as well, so if you’re seldom out of WiFi coverage, then a Touch running Skype and AIM keeps you connected without paying AT&T a dime.

But platforms like the Blackberry and Symbian have also had IM clients for a a long time. The operating systems on these phones can run applications in the background, so they don’t need to rely on a technique like push notification to build SMS replacements. Why then is a viable SMS replacement for the iPhone such a big deal?

I think it’s because of two factors. First, the media love the iPhone and it gets more than it’s fair share of media coverage, due in no small part to Apple’s brilliant marketing. Second, Apple iPhone users seem to make much more use of the web than users of other phones. At WWDC Apple made the somewhat incredible claim that 65% of mobile browsing happens on iPhone and iPod Touch. (Look here for the slide. Scroll down to 11:46 AM). If anyone has real data to support that claim, please leave a comment). I can’t substantiate that number, but AvWx.net, a mobile site with weather for small plane pilots, the iPhone users do have higher page view counts than other platform users, with Blackberry users a distant second.

So if iPhone users desert SMS in droves in favor of free IM, they will be efficient in spreading the word. And if that happens, can the users on other platforms be far behind? …and AT&T can kiss the $625-per-megabyte transport business goodbye.