You have the option - in terms of smartphone OSes - to either do full multitasking (like the N900), no multitasking (like the older iphones) or service multitasking (like the new iphone OS). The latter 2 mean the CPU is free to handle UI tasks exclusively (or almost exclusively in iOS4) so you get a smoother experience.
With full multitasking you still want your apps running so you need to allocate them resources as well, you could reduce those resources but how far? You still want background music/video playing at full speed, webpages loading in the background at full speed so how do you scale those resources?