I for one am actually not too worried about dragons being "common" (if that's the right word for it). Since your character is the "Dragonborn," it makes sense that you will be drawn to and will draw dragons to yourself. The nice thing about dragons is that they are totally outside of the player's control and arena of choice, as well. An Oblivion gate was stationary, hence avoidable, if the player chose to pass it by. In Oblivion, even though you "knew" that Tamriel was being invaded by Daedra, you could avoid entering Oblivion most of the time and the world really wouldn't end.
The dynamic unpredictable nature of dragon encounters in Skyrim should really make the game feel more alive. Maybe at first you will run across one or two here or there. Maybe after killing a few, the dragons begin realizing that *you* are a threat to them, and will start actively hunting you. You might be out hunting for herbs for that special brew and suddenly a black shadow descends silently upon you from behind. You might be about to take shelter in that little cave during a rainstorm, and then those eyes open up in the darkness before you. The difference in Skyrim will be that the player no longer has to choice to procrastinate and avoid his destiny; it will seek him out and his only choice is to face it or flee for his life, if he can.
Personally, I'm hoping that there is a variety of different dragon approaches, ie. ambush, full frontal open assault, you stumble upon one sleeping, etc. In Tolkien's worlds, at any rate, dragons were both violently obvious in their approaches (The hurricane of Smaug's wings) and also capable of subtle ambushes (the dragon from Farmer Giles of Ham, forget its name).
Anyways, I'm eager to see what gamesas has in store.
(I'm also hoping the minimum recommended specs aren't too steep, or it will be 5 years before I can try Skyrim...