How is this not addressed by push notification? I'm asking because I legitimately don't know, and it's reasonable to assume that I legitimately don't know because I've never heard of VOIP outside the context of those big, expensive systems they put in offices.
Push notification would require a custom VoIP setup such that someone phoning you would get routed to a server which then instead of directly connecting you would send a message to Apple's server which then sent a message to your iPhone. The latency is such that your caller might have to wait a long time to connect to you and you're at the mercy of Apple's servers to deliver the push notification. I'm not sure if you could push to multiple handsets either (I use the same VoIP number on three handsets simultaneously). Good luck with that setup as a commercial venture - totally reliant on Apple's push service..
Apart from that, you're then not free to choose any standard SIP based VoIP provider - only the ones Apple approves - or to integrate it with your corporate SIP PBX/Asterisk server.
As I've said numerous times before in this forum, it certainly would be annoying, but the apps I use don't work that way. When I touch the button for one, it takes me right back where it was. Clearly this is something the OS supports, so the onus should be on app developers to do it right.
I would guess you're mostly using Apple's apps which DO multitask or apps that do not require a constant network connection to continuously update data.
There's a few classes of apps where it's not possible at all to restore state. For instance a terminal application which has to maintain a constant connection. Switching away from it, it closing down and then switching back would require a new login. eg. On my Mac I usually have 6-7 terminal sessions open and a few VNC sessions. If, when I switched to read a manual, it closed my session, I couldn't go back to that running copy of vi running on my server to continue editing the code I was looking something up for.
How about a 3rd party navigation app? Today I was in the car with Ovi Maps running on my phone giving me directions and someone rang me. I answered the call, had a conversation and in the background Ovi Maps was still guiding me. I continued to also get email.
That's all fine though. I get it. Apple doesn't make devices for Pros anymore. They're going for the lowest common denominator and don't want to frighten them with features.
It's a bit silly that a $250 SIM Free Nokia does VoIP, multitasks, tethers, syncs wirelessly with my Mac and lets me do my job maintaining servers and Macs remotely but a mega-expensive iPhone doesn't. And the battery lasts about twice as long too.
I've just checked my phone for what it's running now...
Homescreen - like a dashboard showing latest email/messages/calls/calendar/todo
Text messaging
Email
Contacts
Calendar
Location Tracker - provides geo tagging services to any app
SafeWallet - like Keychain
Google - mobile front end to all the google apps
Ovi Maps - Sat Nav with turn by turn.
S60 Web browser - open with a couple of sites.
Gravity - Twitter/Google Reader app
It's connected to 3G permanently, Wifi now and registered to two SIP networks as well as my carrier.
My phone is consuming 0.35W with the screen on. 0.13W with it off. The battery is 1500mAh so that's between 14 and 28 hours use. The battery is a little bigger than the iPhone though, even if the phone is smaller. The iPhone's screen is the biggest power sucker, not multitasking.
Multitasking is not a battery hog and it opens up whole classes of apps the iPhone can't do. It makes things simpler for developers and simpler for users. It's essential for power users. The irony at the moment is that Nokia are pushing Symbian down market into phones that Engadget has problems calling 'smartphones' because they cost 135 Euros unlocked yet they're smarter than the iPhone by far.