See the reference from Android projet : yes, a software using a service (so it is divided in client+server way) can run in background without limitations, in theory.

Anyway, from what I knew and I recently learned, the Android (4.0) "multitasking" has the drawbacks :
- the UI of Android "multitasking" is misleading, showing suspended/closed softwares and not showing running softwares : it is just a list of recent used softwares;
- internal Android "multitasking" <> Linux desktop multitasking;
- Android "multitasking" agressively puts softwares in suspended mode (to save battery and CPU use);
- a software using services can be in background without limitations, but it can be cumbersome for the developer to separate the software in client+server, and the service cannot update the UI (user interface). Many Android softwares don't use services so they are suspended arbitrarily;
- to confirm : Android developer documentation assumes there is no swap memory available, so with limited RAM (128MB-1GB RAM nowadays) of Android devices, softwares are simply closed when the RAM is not enough.

iOS & WP7 "Fast App Switching" are a lot worser than Android "multitasking".