This is actually an option depending on the screen protector you buy. You can get a clear protector which will tend to increase the glare but also allow all the screen brightness through. You can also get protectors that are designed to reduce the glare at the expense of some screen brightness. They generally have a rough surface which disperses incoming (and some outgoing) light. I prefer the anti-glare protectors because they also facilitate writing on the screen since they produce a more paper-like feel.

As far as the N900, all this is academic at the moment. In general you will have to wait a bit to buy third party protectors. Assuming the N900 is successful they should start to crop up soon. I don't know that I'd recommend any of those that already show up on Google Shopping - I've never heard of the companies involved.