Dust under the screen is exactly what it sounds like, dust particles between the glass and the LCD screen below. I have this issue with my HTC made Nexus One and it's driving me crazy. I've already opened up my N1 and fixed the issue once, but the dust has returned and looks terrible, especially in bright lighting (inside or outside). I probably got a bum unit, but I have to admit it's made my slightly leery of purchasing another HTC phone.