Yah it can happen
I have seen a few powerbooks (ibooks seem less susceptible to this) that have about six key imprints in the centre of the screen. It seems more likely to happen on the machines that are carried around frequently. Probably has something to do with pressure on the top a closed pbook pressing the screen against the keyboard.
If you are planning to carry your machine around with you, it would be well worth keeping a screen protector sheet in it (or thin lens cleaning cloth as I do). It will serve well to keep the screen looking fine.
As an alternative, there are also static films on the market to apply over your screen. When these get scuffed up, you simple take them off and get a new one. This option is great if you will be using your pbook in situations where someone is likely to touch the screen. (the name of the supplier escapes me now, but I will update if I find it. . . .)