I had a screen protector for a while, and it built up a few scratches (it wasn't one of the really cheap ones either) so I thought it was doing its job. One day the touchscreen suddenly stopped responding, and I assumed it was broken. I took off the protector to send the phone back, but without the protector the touchscreen worked again, and was much more responsive (despite the fact that when I originally put the protector on I couldn't really tell the difference). Since then I haven't used a protector at all, and I don't have a single scratch, not even a hairline when held up to a bright light, and the touchscreen works a lot better. I've decided that the protector meant I was living with a slightly scratched and inferior screen permanently in return for some insurance against a big scratch, and that that's not really worth it. The screen on the N900 seems pretty tough, I've dropped the phone quite a few times, and I don't have a case for it. The only reason I can see for a protector is if you work on a building site, or live near a beach or there's some other reason you might end up rubbing grit into the screen.

Also, for what it's worth, if you want to put on a screen protector with no bubbles, put on the first edge, then put a tiny amount of water along the join, then put the rest down carefully so the droplet of water moves along as you go. The water makes it impossible for any air to get under the protector and you can just wipe it off at the end. This only works with the static electricity based protectors that don't feel sticky to the touch.