Have a look at what Iain Campbell has to say. Just to entice you to click on the link and read, here's some of the conclusion:
I am no Luddite – I believe in embracing the new technology. I can tolerate PowerPoint for welcome notices, for asking people to switch off their mobile phones, for displaying notices, for displaying our psalms and speaking to the children. What concerns me is the assumption that one need not take even a Bible with one to worship, because it will all be provided on the screen, And what I simply cannot tolerate is the growing dependence on PowerPoint for proclaiming the message of the Gospel, and the niggling feeling I have that we are raising a generation of public speakers who cannot preach a sermon without turning it into a presentation.
Update: Here's another take on Powerpoint and Sermons: 'Powerpoint and the Death of Preaching' from the Exiled Preacher.