Some more thoughts:
One can blow the dust off, but be very careful, blowing with your mouth can leave wet spots (nuff said), blowing with the cans of compressed air has too problems, the air carries a residue which will be left on the surface of the photo, and if you hold the can at an angle or something you can get the fluid to come out and it will freeze the surface with unknown (usually bad in my case) results.
I had a friend that had a small air compressor, with a very good filter system on the input, and she would blow the dust off with a very low pressure air flow.
The key is low pressure so as not to damage the orginal, and if it does not blow off, then just remove it in
PS.
I second the cleaning of the scanner, etc before starting work....
Mike