The big screen and larger work area will doubtless cause a BIG rift between the iPad and the iPhone - the machine is comfortable for multitasking. Whether or not users really know what multitasking is or not isn't the issue; the issue is that users are clamouring for it. The iPhone is already a multitasking machine, concurrently managing many processes and CPU threads. The OS itself isn't that much different from OSX - in that it can and does manage multiple apps at once. But, Apple have put a stopper on the thing for many reasons: battery, security, simplicity. Whether or not they will address this so that users can have a little more freedom is another question. But, users clamouring over multitasking are in some ways, clueless as to what it is.
In any case, AppleInsider have some encouraging points to ponder.
How it could be implemented:
Multitasking in the operating system
A single-tasking operating system, like the original 1984 Macintosh or the Palm OS line of PDAs, is simply incapable of running multiple concurrent applications due to design constraints.
In the mid 80s, Apple adapted the classic Mac OS to switch between apps to create a windowing environment that employed cooperative multitasking. In this model, various apps voluntarily relinquished control of the processor so that they could all appear to run at once.
The downside to this model is that an application that stopped responding could prevent other apps from sharing any processor time, effectively destroying the illusion of multitasking. Modern operating systems like Mac OS X designate a kernel task that preempts other applications so if they stop responding, the kernel can simply terminate them and other apps can continue to work.
The iPhone OS inside the iPhone, iPod touch and the iPad is not only capable of this style of preemptive multitasking, but also employs multiprocessing, which allows different tasks to run concurrently on different processor cores.
For example, when playing back video, the iPhone can spin off the heavy lifting involved with decoding H.264 to a specialized video processor core while the main processor core continues to handle other tasks, such as listening for updates and maintaining the user interface.
Apple's secretive A4 processor in the iPad is actually a System on a Chip that incorporates multiple processor cores, each of which can handle simultaneous tasks.
Multitasking in a mobile environment
While there are certain benefits to being able to run multiple applications at once, there are also a series of reasons why Apple deliberately chose to use a security model in the iPhone OS that only allows one app to run at once.
The first is limited resources. Mobile devices have a finite amount of RAM and CPU power, so allowing multiple apps to be loaded and running all at once introduces new problems related to the mobile device's performance and battery life. It also increases the system's complexity, as users will now be forced to monitor and manage the processes running in the background.
Another problem relates to security. If apps aren't simply terminated by the user in a straightforward way as they are on the iPhone OS, users may not be aware they are still running. Background apps might provide a valuable service, but without any restrictions, they're also able to install spyware, viruses and other malicious software. That's simply not possible to do on the iPhone OS.
The multi-billion dollar malware problem on the PC desktop illustrates why unfettered software installation has both pros and cons. Even background services that seem harmless, such as Google's Latitude, can expose users to unapparent security threats if they don't have the security credentials to recognize the potential risks involved with, for example, advertising their location.
For more, check the entire AppleInsider article