keep in mind from what the videos show to come up with some info you have to have apps like yelp installed. so you have the siri stuff doing its thing while bringing up the other app to do its thing. I am betting it is using the dual core more of apple being lazy then anything else.
ON my iPhone 4 i cannot listen to music on the iPod app while downloading songs from iTunes app . (the music stops every time a song finishes downloading and goes to processing. I am thinking that sure requires probably more cpu time and apple rather just sperate the siri and 3rd party apps on their own cores then actually making it to run on the 4.
Like i said I think its more of a lazy developer thing .
With playing music and downloading songs it quits because you are writing to the music storage area. That doesn't have anything to do with the A4, or the A5. It's been this way since the very first iOS device. I have a jailbroken iPhone that allows me to use the phone while it's syncing to iTunes and I can't view pictures while putting photo albums onto it. The app won't open or it says "rebuilding library, please wait." It force closes the photos/iPod apps until it's done syncing and even then you have to restart the photos\\iPos apps after it's done. That's true of any Apple app that syncs data to the phone. The photos/iPod apps do this every time you sync content to it, the icon is greyed out when you disconnect and open the multitasking drawer. That's a read/write issue and all computers do this to some extent. You can't write firmware or other critical software updates in OS X while it's running either, you have to restart and it updates it then. That's also why a non-jailbroken iPhone locks the screen while syncing. All of that behavior prevents the user from terminating the process mid-write and corrupting the data, not Apple being lazy.