If you haven’t done so already, you should get out the iPhone cable and update your carrier settings. The long awaited MMS is finally here, delayed for what seemed an eternity by the ever-failing AT&T.

For the update to take effect you’ll need to reboot your phone. From there, though, you’ll have a nifty little picture icon next to the message line of every text. The service allows you to send pictures and video at no extra charge, assuming you’ve already got some sort of bundled plan.

AT&T has announced plans for the iPhone MMS rollout via its Facebook page. We’ve known for some time that the service would be coming on Friday, but the how and when wasn’t so clear.

From the company’s Facebook page:

MMS Update: We know you’ve been eager for this service so we wanted to offer a quick update on the launch plans for MMS on Friday, Sept. 25. Late morning, Pacific Time, on Friday, the new carrier settings update enabling MMS should be live and ready to download through iTunes. We’ll provide the steps and all of the details you need right here at that time.

For my fellow east-coasters, that means we’ll be waiting until early afternoon before plugging and unplugging our phones for updates. Some people are speculating that AT&T is nervous about the update, and rightfully so. This is the kind of update that has been anticipated for so long that it is likely to kill AT&T’s networks in a lot of places. And if the service fails on day one, the public will be calling for the bodies of AT&T executives.

The only way I can think of AT&T making the wait for MMS worse would be charging customers extra for the service when it finally goes live. Luckily, it doesn’t seem like they’ll be doing that…to everyone.

In the AT&T iPhone 3GS FAQ PDF the company states in plain language that MMS is coming soon “at no extra cost to customers with a text messaging bundle.”

That plain language leaves some room for interpretation, though. AT&T says they won’t charge you extra for the service, as in no extra fee for sending the messages. But how will they count against your SMS bundle count? If, say, you have the 200 messages plan, which costs $5 under AT&T, will each MMS count as one message, or will they dock your total more for sending videos of your son barfing on the dog?

For now I’ll hope they don’t turn their MMS policy into a “credit” system, whereby they can alter how many credits you get charged per message. We already know that texting plans are a racket. Going to credits could send furious texters into a frenzy.