AT&T will finally provide MMS for iPhone on September 25.

AT&T said it would finally be providing iPhone 3G and iPhone 3GS customers with multimedia messaging capabilities via a software upgrade Sept. 25.

The MMS capabilities were one of the most-heralded features of the iPhone 3.0 software because it plugged a nagging hole in Apple’s mobile platform. Rivals like Research In Motion (NSDQ: RIMM)’s BlackBerry, Windows Mobile, and many entry-level phones have been able to send MMS messages for years.

AT&T didn’t offer this messaging capability when the 3.0 software was released in June because it wanted to ensure its network could adequately handle the traffic. This has garnered ill feelings from some iPhone owners, as well as a class-action lawsuit that alleges the second-largest U.S. carrier misrepresented the iPhone’s capabilities.

“The unique capabilities and high usage of the iPhone’s multimedia capabilities required us to work on our network MMS architecture to carry the expected record volumes of MMS traffic and ensure an excellent experience from day one,” AT&T said in a statement. “We appreciate your patience as we work toward that end.”

While AT&T technically will miss their “end of summer” deadline by a few days, the move may help it squash growing complaints about the carrier’s quality of service from a small, but vocal, group of iPhone owners. AT&T has been investing heavily to improve its 3G network, and it is also deploying 850-MHz spectrum to improve coverage in major metropolitan areas. Improvements in service could help AT&T retain iPhone subscribers even after it loses exclusivity of Apple’s popular smartphone.

The Sept. 25 software update will be available through iTunes, and it will not be available for the iPod Touch or the original iPhone. The carrier also said it would be offering tethering in the future, but did not get into specifics about the launch date or pricing.