Apple unveiled the 4th major version of their iPhone OS (and iPad and iPod Touch) this morning, touting over 100 new user features and over 1500 new developer APIs.

And yes, you've been waiting for it for years, and those of you with weird, irrational hatred for the iPhone will need to find a new target for that weird, irrational hatred.. that's right.. custom wallpaper!

What else?

Multitasking! — Yup, iPhones will be able to run more than one app at a time. A double-tap on the home button brings up a little dock-like interface showing all running apps. Apple makes a big deal about "doing this right" so it won't have as dramatic an impact on battery life and performance as it tends to have on other platforms. It sounds like they're doing this by providing specific APIs for doing certain background tasks. In other words, the whole app doesn't keep running, using all the memory and processor cycles it would be using if you were looking at it. The new background services include background audio, VoIP, background location, local notifications, and a system that suspends an app instantly so it can resume where it left off when you switch back.
Folders — Organize apps into custom-named folders.
Unified Inbox — See mail from all accounts in one inbox, without that brain-killing 5 tap dance I've now done approximately 9 billion times.
iBooks on iPhone — Yawn. No surprise there, though keeping your place in a book on iPhone and iPad closes a big gap with the Kindle.
Social Gaming — Wow. Apple basically now has their own XBox Live. Achievements, leaderboards, matchmaking, etc. That's going to be big.
iAds — Apple's own built-in mobile ad network. Wow. Ad interactions will stay in the app, Apple will sell and host the ads, and 60% of revenue goes to the app developer. If this works, Apple may have just opened the floodgate to tons and tons of mobile advertising dollars, elusive up to now.

These features close a lot of holes in the iPhone OS, adding tons of things users will love as well as several new models for app developers to make money. This is a big one.

Apple usually posts the video of these events soon after they end, so that should be available soon.

A developer preview of OS 4 will be available today, and the update will ship this summer.

Update: The new OS will be available for all 3G and 3GS iPhones, as well as recent iPod Touch models, though only the 3GS (and later, assuming they announce new hardware this summer) will support multitasking and some of the other advanced features. OS 4 will be on the iPad in the fall.