Yes, the newest iPhone 4 OS allows multi-tasking... but not on all applications. A HUGE difference between iOS and webOS is the fact that webOS is built for this feature. But with iOS, it's actually up to application developers to enable the ability to multitask within the app itself (and to do so, they have to follow very stiff rules set by Apple in doing so)...and only a small fraction of iOS apps have been updated by the devs to add multitasking functionality.
Games like Angry Birds and Plants vs. Zombies are built according to the iOS multitasking rules, so you can stop and resume them when moving between functions. But with an application like YouTube, iOS will not truly multitask like webOS. With iOS, to have a YouTube video load you have to have the app fully open and be staring at that screen. Unless you have that application fully open, the video cannot load. In contrast, webOS multitasking allows users to have apps like YouTube load in the background without the application having to be the main focus (you can be on another app in the meantime) - you can't do that with Apple.
To quote TechWorld: "Let's be clear about what iOS4's multitasking is: Rather than run multiple apps at the same time, iOS4 freezes the ones that aren't in use, with the exception of certain functions. Music, for instance, can play in the background, VoIP apps can answer calls and GPS apps can give directions.But most apps won't do anything except go to sleep, which means one of the classic tricks of multitasking, loading one task while you perform another, is not available unless the developer adds that function under a special task completion API. Some apps, such as Flickr, may take advantage of this feature for large file transfers, but others won't. Waiting for a YouTube video to buffer over a 3G connection? It won't go anywhere unless you're staring at the loading screen.
...What bothers me the most, however, is the sloppy implementation of iPhone multitasking. Every time you open an app, it gets added to the tray, and the only way to close it is by pressing and holding any app icon, then clicking the top-left corner of the apps you want to close. If you don't micromanage, the tray quickly becomes overrun with clutter, making it hard to find the apps you really need."... not to mention a hard drain on the iPhone battery when multitasking is enabled.
Multi-tasking means I can perform multiple tasks via apps at the same time. What iOS does is put "most" apps into sleep mode (or freeze it) while running one app in the foreground. This is called fast-app-switch, not multi-tasking. Apple is performing slight of hand with many apps, and multi-tasking (fast app switching) with only a small percentage.
Differences in multi-tasking aren't as night-and-day as they were last year, but there are still some little areas (like above) that stand out like giants when the 2 systems are compared.