One correction: The "multitasking" in Windows Phone 7 is actually going to be a lot more sophisticated than what any version of the iPhone OS has. Applications will be put to sleep when you switch away, so if you come back they'll pick up right where you left off instantly. The apps won't have to re-load and re-initialize. If the OS determines that it needs the memory resources used by an app, it can close it and its state will be preserved. Currently the design won't allow 3rd party applications to run in the background, but it does allow them to be paused in the background for quick wake-up. Not a perfect solution, but better than any other non-fully multitasking phone OS that is available now.
On to Apple...
Apple could come up with a clean way to do multitasking on the iPhone if it really wanted to. The concerns about battery life and poor performance can be addressed, and the iPhone OS already has the tools to make it happen. Up until now it has just decided that they don't want to do that.
One potential solution to this is to require app developers to put processes that they wan to run in the background in their own threads that don't get closed when a user goes back to the Home screen. Restrictions could be placed on those threads, such that they are only allowed to use a certain percentage of available CPU resources, and a certain amount of RAM. These would address both battery life and performance concerns.
Most of the time when people talk about wanting to do multitasking on a phone-sized device, they're not talking about having multiple apps on screen at a time; they want to do things like listen for incoming instant message conversations (or keep them active while using another app), listen to music in the background while doing something else, making a VOIP phone call, or something similar. Those things don't require a full screen GUI or 100% of system resources to keep them going. A small, lightweight thread could pretty easily handle these tasks in the background while a user performs other tasks on the device.