What's wrong with running "hunting the dragon down and catching it unawares for a fight" as a skill challenge? If the PCs lose the skill challenge the dragon ambushes them and still the fight.
I was going to suggest something similar, except to run the "start" of the fight as a skill challenge. Something like:
Set Up: The dragon is circling above, making periodic strafing runs with it's breath weapon. You need to avoid its attacks and lure it to the ground to fight it.
Primary Skills: Bluff and Intimidate (to trick or enrage the dragon to land and fight in close quarters), Acrobatics and Athletics (avoiding the strafing runs), Perception (finding suitable shelter from the breath weapon)
Secondary Skills: Insight (figuring out how to entice the dragon to land - provides +2 on next Bluff or Intimidate check), History (remembering the details of the last hero/party that fought the dragon), Nature (helps with finding suitable terrain features to hide in - provides +2 on next Perception check)
Complexity: 1 (4 successes before 3 failures), so each challenge is short, but make the DCs high for the PCs level (so they are likely to fail a couple before succeeding).
Success: The PCs manage to lure the dragon to the ground and normal battlemat combat begins.
Failure: The PCs each lose a healing surge (maybe two - you want this to be the big climax after all) from being caught by the dragons attacks, and the skill challenge begins again (unless the PCs choose to retreat)
That's just off the top of my head. You could probably play around with the skills a bit and maybe find some that are more or less suitable.
