Oh my goodness. Absolutely not! Why on earth would I want to warn someone who is potentially breaking the law, putting their own and the lives of others at risk, that there is a speed camera ahead and to slow down!? The idea just doesn't compute to me.
Don't encourage people to break the law by warning them there is a speed camera. Sure, they may slow down to pass it, but I promise you they will speed up again after passing it.
Do us all a favour, don't warn the pricks. Let them get caught and face the consequences of their actions. Intentional speeding or not, I don't stand for it.