I have been using a screen protector since day one, I got it free with a protective case from eBay and I've had no problems with it. I would not want to use my n900 without one, it's too valuable and vulnerable and heavily used for me to take chance.

I'm also interested in getting one of the privacy screen protectors, the ones using the 3M microblade technology. I did actually get a cheap one from eBay but it's orientated incorrectly, someone sitting beside me on the train would be able to see my screen just fine but someone sitting above me wouldn't. I assume the manufacturer didn't realise that the n900 is used almost exclusivly with landscape orientation. If anyone here has found a correctly orientated privacy protector then I'd appreciate them posting the details here.

On a related note, it took me a while to find a decent protective case that was usable. I tried a soft rubber case and a hard plastic case and found major issues with them both. I now have a 'gel' case which I'm very happy with. It's worth noting that the gel case has an integral screen protector, it's transparent and the portion covering the screen is much thinner than the rest of the case. It's thin enough for touch to work although I found the accuracy suffered, that may be because my conventional adhsive screen protector was still installed under it. I cut the screen section out of the case with a scalpel but it may be an option for some people and as a case it's wonderful.