A recent patent filing indicates that Apple is getting closer to adding its own instant messaging client to the iPhone.

The filing is titled "Portable Electronic Device for Instant Messaging," and covers methods for sending, receiving, and viewing ongoing conversations. The proposed GUI is similar to Apple's current interface for SMS.

Built-in support for IM has been missing from the iPhone, to the dismay of many users. The patent filing shows Apple has IM on its mind.

At the recent launch of the iPhone SDK, AOL demonstrated an AOL Instant Messenger client, but it isn't allowed to run in the background, which normal IM applications rely on.

Support for IM is quickly becoming a must on mobile phones. Users want the same thing on their mobile phone as they have on their computers including IM, according to Leif-Olof Wallin, research vice president at Gartner.

"It will be a blockbuster in two, three years," said Wallin.

But instant messaging is also a threat to the massive amounts mobile carriers make from SMS messaging, which the iPhone currently supports.

"They can charge much more for SMS compared to IM," said Wallin.

Pressure from its carrier partners is a possible explanation for why Apple has taken a hands-off approach to a feature many users want, according to Wallin.

"At the same time it can't afford to be left behind," he said.