iPads have a stage display app for propresenter (assuming you're using propresenter and aren't already using the stage display function for your worship team) its a bit of a stretch I know, but I thought it may be worth mentioning. We use an IM client to communicate accross the building. There are lots of clients available for smart phones. Actually, I remember using IM clients on my flip phones way back when. So it may be a pretty easy to implement solution.