No I don't announce myself. I've found the 'sneak attack' method works best. If I'm riding on the local MUP at dawn the few folks that are out there are usually working out (jogging, power walking, rollerblading, bicycling) and seem to be quite aware of their surrounds, so no need to say anything. If later in the day I find most folks either are in packs (walking, jogging, bicycling, etc) and are either engulfed in conversation with their mates, or they are wearing earphones that prevent them from hearing anything.
FWIW: I'm not the fastest rider by far (my bicycles are best described as 'touring' rigs), so I only occasionally pass another bicycle rider.