Unfortunately, you can't use MIDlet to control phones' native functions (such as make a screen saver or read messages from inbox). Midlet runs in a security mode, which not allow to access phones' native resources.
Another apparent example is Nokia SMS API. You can use this API to detect whether phone has received some messages through some ports and meanwhile there should be another midlet working in a client mode only send messages to those ports. But you can't use SMS API to detect whether there is any new message in phone's "Inbox".