I agree with the post above - sounds more like a dead pixel than a spec of dust.
Check this out: one night I noticed a freaking ant behind the screen towards the bottom of my 23-inch Cinema HD! He was having a hard time moving around - who knows how the heck it got in there. Anyways, I shut off the display went to bed and put in a little prayer that he'd be gone in the morning. Much to my surprise he was
. I laced the back of my desk with ant spray so that (hopefully) won't happen again...