Why the long wait? According to AppleInsider's sources, the problem with multitasking is not technical. The two biggest concerns, it turns out, are security and battery life. It reports, "This behavior is prevented by the iPhone OS' security model, which is designed to close the current app whenever the user returns to the Home screen or accepts an incoming call. This design prevents apps from being able to run in the background without the user knowing, and therefore erases any real potential for spyware, adware and viruses."
Believe it or not, iPhone OS 3.x already supports multitasking. Applications such as SMS, email, the iPod, the phone and others can function while other apps are being run. Apple has restricted multitasking only to apps it deigns worthy of the functionality (more heavy-handedness from Apple?).
Apple has been working on a way to allow third-party applications run in the background without sacrificing security and battery life. One factor Apple needs to work out is how users will switch from app to app easily. Right now, any idiot can press the home screen button and be returned to the iPhone's home screen, where they can access other apps. Perhaps this is why the iPhone OS 3.2 SDK now includes the Long Press gesture control. Apple might give the home screen button new powers. Pressing and holding it, for example, could bring up an app switcher of some sort.
Unfortunately, the new operating system has "a long way to go" before it is ready. Does that mean it won't be ready in time for what many expect to be a new iPhone launch later this year? Is this why the iPad, set to launch in a few weeks, will run iPhone OS 3.2 and not iPhone OS 4.0? Only Apple knows.
By the way, anyone interesting in seeing purported spy shots of the fourth-generation iPhone should check out this link here. Looks like these were shot in the bathroom of some dank industrial building over in Asia somewhere. Enjoy.