I dont think any of this really needs to be added to the iPad, it does not need full on multitasking, only basic multitasking, let me explain.
iPhone OS already HAS multitasking support, however this is just for the Apple core apps such as iPod. Thanks to this we know it has the power to run for example iPod + another app.
Now do we really want, especially on a device like the iPad that was designed, lets face it, for the more casual use of browsing, using a few apps etc, a complicated task management system. We are all computer literate, people like my Dad who has a laptop to use the internet doesn't need to know where things are in the background etc, and that is the bueaty of this device. Its that fast he can be browsing > hit home > open pages -> finish in pages, hit home hit safari and he is browsing again, simple. Commands for expose, task managment applications are NOT needed. This is not Windows, Linux or Mac OSX.
So we know that apps can already run in the background, all Apple needs to do is allow developers to do the same. It would be reviewed upon submission and if an application such as a game has it enabled it gets rejected, because there is no need for it to run in the background (save state->home(quit)->do other tasks->back to NFS, resumes from saved state).
So what apps should be allowed backgrounf privaldges? Audio apps. Simple, give audio apps like radio apps the ability to run in the background. The system can be smart about it and because it knows an AUDIO application is running it automatically kills 'iPod' (or the previously running audio application) and runs background (even when paused, just idles like iPod) until another audio application is opened. This way it acts in a way users are familiar with in the way iPod acts, it doesn't get confusing to the user wondering what apps he has open, and it doesn't bog the system down with too many applications being needlessly opened.
Now they could do the same for IM apps and give them a specific background process ID and they behave similar in they run until another application doing the same job is opened. But this isnt even required, if unobtusive notifications get added Push could just be used instead.
Example.
User is listening to iPod and surfing safari. Decides wants to listen to web radio. Opens mlb.tv and starts a radio stream, iPod detects this is the now active audio app and closes. User returns to safari and browses listening to the game, user then gets a notification (maybe using a notification bar more like the pre?) that he has a new IM, hits home goes to IM app (or hits the notifation tab) safari closes IM opens, he types and then hits home back to safari.
Simple. All of a sudden with just backgrounded audio and push for IM(email) etc and a forground app its like you have 3 applications running.
This is not like a PC where you will do things SIDE by SIDE. You have ONE app displaying the whole screen at once in fullscreen so there is no need for MULTITASKING, just basic background support (for applications that NEED it). On a PC you may have a word document open with a browser next to it and a video playing 'on top', that isnt possible on a device that shows one fullscreened window at once, so why support multitasking which will allow videos to be played in the background when you are in another app and cant see it, it will drain battery and slow down the machine. The ONLY advantage your suggestions add IMO is that it would speed up switching regularly used applications and that while being a bonus is not enough to slow down the system and make it more confusing to other users as the hit home hit icon way of switching is fast enough. Plus hopefully apple will allow more than 4 applications on the dock so no matter what home screen you are on those regularly used apps are easy to reach.
