The hardest bit of multi tasking seems to me to be the implementation. At the moment hitting the home button to quit is so simple. If they allow multi task how will they distinguish between switching apps and quitting apps.
i dont think they need 2 years to think about the solution to the question you posed. Palm Pre already solve that in the most elegant fashion, even if apple and MS can't copy it, its not difficult to find an alternative way.
Although I wholeheartedly support multitasking, the design philosophy of wp7s is opposite to the approach of "apps", it designed to blur the boundary among different "apps", and offers approach of "tasks" or "hubs" instead.
So, when number of apps become less important, the call for multitasking will also be less.
But i can hardly imagine this last for too long, people nowadays wants unlimited devices. The design of wp7s maybe extremely edgy and fun, but its design is also a limited device, its function will have to go beyond what it shows so far, in which case, both number of apps and multitasking will need to be implemented.