Compressed air cans, and many vacuum cleaners for that matter, don't remove dust but just move it around.

Dust is somewhat conductive so as it coats the surfaces in a computer it can "short" exposed traces or wires. There should be no permanent damage once the dust is removed.

Be careful not to use too much suction or compressed air pressure. You don't want to suck a wire or jumper out of a socket!

Too see some really dirty pictures (of computer innards) try this story from an IT industry publication: Dirty, dirty PCs: The X-rated picture guide.