Hi,
Will it be possible to create a unity app that runs in the background with Unity 3? (like for example having something that sends GPS coordinaes continuously).
Thanks
applications can not run at all in the background.
there are 5 types of micro tasks that can be performed in the background but general applications are paused until they come to foreground again. (known as fast app switching)
and that part is working out of the box already with Unity iPhone 1.x 
Did I misunderstand what iOS 4 Multitasking feature is?
And what about Android multi tasking? All I am wondering is if it is any way possible to run some (very small) code with the app is switched in the background.
Any pointer to some doc would be useful.
Yes you likely missunderstood it.
iPhone has no true multitasking.
The android has it, but I am unsure if it works with NDK applications and its clear that the drain on cpu and battery are there…
Either way, you would definitely NOT do such an application with Unity, because its a heavy thing targeted at immersive 3d applications, not lightweight background services.
For the iphone you would use one of the task types + xcode to do such a thing, on the android java and a lightweight application