I'm going to strap my n900 to my wrist to be a permanent cyborg :-)

seriously, a simple/thin leather sleeve and probably a decent ultra-clear type screen protector.

I still use my n800 in the cloth sock provided, with a bit of velcro stuck to the back of the n800 so that it doesn't slide out too easily, and only recently replaced the factory-fitted screen "protector" with a reasonable quality replacement (it was looking pretty cloudy!)