Apple has launched the second iteration of the iPhone to the sort of frenzy the company has learned to incite. Along with the ability to access 3G mobile networks, the iPhone gets a major software update that includes the ability to download third-party applications from an online store called simply "the App Store."

If you've used a BlackBerry, Windows or Palm mobile device, you might be wondering what's so special about getting to download apps to a smartphone. I'm not here to answer that question because this site is not "Some OS X Carpetbagger's Deeply Conflicted Thoughts About Apple's User Experience Management Planet." Please just take my word for it that getting to download apps to an iPhone is a pretty big deal to people who own iPhones (or iPod Touches, which run the same software) and who were not willing to "jailbreak" them months ago.

The iPhone's basic software layout is pretty nice. Five minutes with the mobile version of Apple's Safari Web browser on a free iPod Touch was enough to make me resent my BlackBerry. After a day with it, I became thrilled that my BlackBerry had seemingly died of shame, choosing to starve itself to death by way of a bad recharging circuit than live in a world where its formerly doting owner was counting the days before he'd be eligible for a subsidized iPhone.

Besides Safari, the iPhone comes with the usual stuff: address book, mail client, calculator and notepad; and it also has a YouTube player, a map program and some lesser applications. One thing it has been missing, however, is an instant messaging app.

That particular lack was noticed fairly quickly when the iPhone first launched. Some wrote the matter off to Apple acquiescing to its partners in the telecom industry, who'd prefer mobile users stick to SMS , which is more lucrative for carriers. Others subscribed to the less conspiratorial theory that Apple simply didn't have "mobile iChat" ready to roll out at launch.

Whatever the reason, Apple provided no IM app and no way for anyone else to write one unless it was delivered as a Web application. Meebo and others have shown that Web-based IM works, but the iPhone posed a second challenge by forbidding applications to run in the background.

It might seem like an arbitrary restriction, but it makes sense when you consider that even relatively powerful desktop computers can be brought to a halt by misbehaving programs, or a proliferation of running applications. It also helps curb power consumption.

That limitation means that the "instant" part of instant messaging is ruled out if a user is, say, looking up an address or watching a video on the device, because an IM client would have terminated when the user ran another app. It also becomes harder to convey presence information, because a suspended application can't report whether the person using it is idle, or still connected at all.

Last month Apple announced a push service that offers the possibility of apps that can suspend in the background yet still provide a largely real-time IM experience. The service will allow the iPhone to continue to exert a level of control over which processes are running, while providing 'net-connected applications a way to get the data they need and respond to it.

All of that goes toward explaining what iPhone users exploring the new App Store are discovering: Of the major IM networks, AIM appears to be the only one with an iPhone application.

Looking over the FAQ for the application, it's clear that the client is laboring under the same limitations any other application or Web-based IM service faces on the iPhone: If the user exits the client, AIM's servers will keep the login session active for about five minutes, putting any incoming messages in a queue, before deciding the user isn't coming back and quietly discarding the messages it queued up.

The FAQ also notes that this situation will change in "a future version," surely referring to the upcoming push service, set to launch in a few months.

Despite those limitations, we'll be reviewing AIM's client along with a few offerings for Twitter fans and Facebook's app, which promises access to its chat service. I'd have loved to offer a quick look today, but Apple's talent at whipping up consumer demand seems to have outstripped its capacity for meeting it: I'm still waiting for Apple's overloaded App Store to come back online.