iPhone, for specific features using the built-in ipod functions does give the appearance of a multi-tasking device, but that same functionality is not availble to all your applications. You need to hit the home button and then relaunch the app. However, this can be added with updates in the future as Mac OSX does have this feature and Apple does claim the iPhone OS is a form of MAC OSX. But it's not presently there.
The iPhone uses OSX with a different shell. It definitely can and has multiple processes running like any Unix system and in fact can support multiple users as well. Apple has said multiple times why they artificially limited third party apps from running in the background, battery life and stability. Windows mobile and Android both suffer from being able to load a bunch of third party apps up and just one poorly written one can drain the battery in short order or make the device feel sluggish by eating cpu cycles silently. If you ever used Window Mobile I am sure you have used task killers to clean up background apps. For us technical users we can deal with this process management and tracking down a rogue app, your average non technical user will not understand what a process is or why the battery is going dead, they blame the device since there is no obvious way to determine which third party app is at fault nor do they care they just want it to work.
I have hope Apple will implement some way for at least one or two third party apps in a future update, but they may not unless they can be satisfied that the battery life can be controlled properly. I imagine if they did it would probably be some even more strict approval process in the app store where they test the app over a longer period of time for background stability and power consumption.
The impact on home automation is that if a native app is devloped (not just simply browser access to a HA webpage) and you are task switching around to different applications, you won't be able to get back to the same spot without drilling back down again. Perhaps not a show stopper, but it does have an impact.
This is not true. Apples SDK is very clear in recommending that app's should save their state when the close message is sent to them, then reload that state when reopened. Now it is up to the developer to implement saving state and loading state, because for performance reasons, the brute force method of saving and loading all of an app state in ram to flash would be too slow compared to just a few kilobytes of true application state such as where you are in a menu structure etc that only the developer knows and can decode properly.
This does play into multi-tasking, the original iPhone only has 128megs of ram, the new 3gs only 256megs, iPad is unknown I would guess 256 or 512megs. With 128MB of ram the iPhone only has about 40MB free left for a third party app after the OS with its background processes loaded(phone,sms etc.) and no page file. In order to multi task third party apps, they would need to all fit into 40MB of ram at once. Again if you ever used Window Mobile I am sure you have run into the common out of memory errors that plague it when you loaded too many apps at once since it also had such a small amount of ram, I believe Android tries to kill apps in low memory conditions. Apple avoided this problem completely, everything about the design is resource conservation, to keep cost down and battery life up, leaving persistent state up to the developer, since the developer knows how their app works.
As for flash pages in the browser? This is an issue. Negating it buy saying may sites have non-flash content means for some that they'll need to redesign their own home browser configuration if they had used lots of flash based content.
Apple is purposely ignoring Flash. They do not want a third parties code running in their browser possibly destabilizing or creating an exploit in it and the user blaming Apple for it while Apple is waiting for Adobe to make a fix. Apple along with Google are pushing hard for HTML5 which will cover what Flash is needed for today (Playing video, vector graphics, canvas, etc.) so they can control the code base that implements it.
This maybe a mistake on Apples part since flash is so entrenched, they are gambling their mobile user base has enough clout to push websites to design against standards only and make apps for special features. I bet the CEO of NY Times was asking their technical staff why their webpage was broken on the iPad after the keynote demo and they had to explain they difference between Flash and html and why they use it.