I've had a Zagg invisible shield on my N900 since I bought it. If you install it correctly, there are no bubbles and the protection it offers is astounding. It's been on my phone for a year now with no peeling or signs of fatigue.

I've demonstrated it to friends before by taking my keys and slashing at the screen a few times, or a beer cap...

In fact, the LCD broke one drunken night and there was no damage to the external display at all, so I was able to just replace the LCD.

I would recommend BestSkinsEver.com because they're cheaper, but they don't have a N900 cutout available -- however they use the same material that Zagg does to make their protectors.