Hi, I have the same identical problem: I hear the nokia startup tone, if someone call me the phone ring just one time and close the call , and the screen is ever black. So I have brought the phone to a nokia laboratory to fix it, and the answer was:
"we have found water in your device, hence your phone is not more under warranty"....(this is impossible, i have never left my n900 near any water....)
So the guy to prove it, he gave me this photo:

http://www.alexdiclaudio.eu/n900.jpg

I don't know how is possible, but I don't believe that the water magically appears into the center of my phone, leaving perfectly cleaned all sides...

Is there anyone that know if they are trying to trick me?

Thank for the help guys