The iPod 3G and second-generation iPod touch are also supported, to a lesser extent, and an iPad-compatible version will arrive this fall, but those using the original 2007 iPhone and iPod touch models are out of luck. To download iOS 4 open iTunes (version 9.2 is required), plug your iPhone or iPod touch into your computer, and click on "Check for Update." The update is free for everyone to download, including iPod touch owners, unlike previous years.
Remember to backup your data just in case something goes awry, or if you want to be extra careful, you might want to consider holding off on updating for a day or two to make sure that there aren't any major issues. Last year, some early adopters of iPhone OS 3.0 found that the update was hurting battery life, or that they couldn't reactivate their phones on AT&T's network after installing the update, leaving them restricted to emergency calls and Wi-Fi Internet access.
If and when you do take the plunge, check out Pandora's updated iPhone app, which was one of the first to hit the App Store this afternoon with the promised multitasking support, allowing you to stream music in the background for free. Will you be updating your iPhone or iPod touch today? Let us know how it goes in the comments.