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(1:21 p.m.: According to multiple report... s and one phone call from a friend, the update is now available, with download sizes ranging from 315 to 579 megabytes, depending on your model of iPhone or iPod Touch.)

For those who own the most recent eligible hardware--the 2009-vintage iPhone 3GS or 32- and 64-gigabyte iPod Touch models that began shipping late last year--the big reward will be multitasking, or a reasonable facsimile thereof. While iOS 4 still won't allow third-party programs to run alongside each other and Apple's own programs, it will invite updated versions of them to hand off common background tasks--such as music playback, tracking your location, waiting for an incoming message or Internet phone call, finishing a file upload or download, or saving their current state--to the operating system before suspending their activity.

That's a little less than what some other smartphone platforms allow--or what I've seen jailbroken iPhones do--but it's likely to be good enough in practice.

Apple's upgrade will also let users group applications into onscreen folders; offer a "universal inbox" view of e-mail in all of their accounts; and shop at and read electronic books downloaded from the iBooks store that debuted on its iPad this spring.

As with multitasking, some iOS 4 features--for instance, setting custom screen wallpaper and connecting Bluetooth wireless keyboards--won't work on the iPhone 3G or most iPod Touch models. (Note that this time around, Apple isn't charging Touch owners for this update.)

Apple has its own summaries of what iOS 4 brings to the iPhone and the iPod touch; for more detail, see AppleInsider's breakdown of the changes. Over at MobileCrunch, John Biggs offers some sage advice about this update and the Apple upgrade cycle in general.

On that note, if you own an iPhone, you might want to resist the temptation to upgrade to iOS 4 as soon as possible. Last summer, some early adopters of iPhone OS 3.0 found they couldn't reactivate their phones on AT&T's network after installing the update, leaving them restricted to emergency calls and WiFi Internet access. Considering the odds of history repeating itself, it might be wiser to suffer the indignity of running older iPhone software for an extra day.