It might be stuck on the finder screen in the mirror box. If so a quick blow from a Rocket Blower should be enough to shift it. On no account rub the screen surface as these are quite delicate as a rule and easily damaged. It's worth getting a Rocket or similar anyway as they are a good way of getting rid of excess dust (though I think the D90 has sensor self cleaning routines built in).

If it is inside the finder chances are it'll eventually move or disappear as what gets in can usually get out again...