Mine is doing the same. While outside, not with direct sunlight but with a very bright cloudy sky, the n900 screen would become black while calling. Works fine indoors.
I dont use a protective film, and i tested a friends n900 and it was working fine under the same conditions, so i guess it might be a flaw from my proximity sensor :/
Only bought it 3 days ago, so i am exchanging it for another, lets hope it isnt worst than the one i have now xD