Let me say that I don't have an N900 and I don't use a screen protector on my N810 but I have used a variety of protectors on my various Palms. Based on my experience, let me lay out what options you should be considering.

The first thing you have to decide is whether glare is a problem for you. Screen protectors can be clear or glare resistant. The latter have a rough surface which disperses some of the incoming sunlight to reduce glare. Unfortunately they also disburse some of the outgoing screen display light so your screen will seem dimmer. I, personally, like the glare resistant version but that is due to another characteristic (they have a better feel when writing on the screen with a stylus) that will probably not be a big consideration with the N900.

Screen protectors also come in lighter and heavier weights. For PDAs, heavier weight protectors are easier to install and last longer. Boxwave was my brand of choice in a heavy weight protector. However, I suspect that a heavy weight protector may affect the touch sensitivity of the N900. If this is the case, a lightweight protector, such as the Fellowes, might be a better (and cheaper) choice.

There are also screen protectors which claim to be removable, washable, and reusable. I have not had good experiences with any of these. I would recommend leaving the protector on, cleaning it with a microfiber cloth, and replacing it when it becomes noticeably scratched.

If it were me and I was dealing with a $600 device, I'd buy a pack of light protectors and a heavy protector and try them both.