hey. i opened my phone and changed the casing. and then after switched it on there are like 8 very small tiny black dots accross the screen. please help?

Dust? It's pretty difficult to keep that stuff out once you expose the LCD.

Also depending upon how much you stripped down the N900 and handled the parts, you may well have damaged the screen too - if the 'black dots' are pixels, it's likely they are now dead unfortunately.