OK, so my 5 13th lvl PCs faced off with the 17th lvl elder white from the MM.
First they fought the white's blaspheme guards, and only used 1 daily attack power and a daily utility in the process. The battle drained quite a few healing surges (those dudes are nasty, thanks Open Grave!) but they saved most of their big guns. After descending into the dragon's frozen cave they then fought another group of varous blasphemes and a living breath weapon. Once again they hung onto their daily powers and stuck to their at-wills/encounter powers.
Since the white dragon was the last fight for miles around and nothing was stopping the PCs from resting after fighting him I knew if I gave them another rest the fight would be pretty easy despite the level gap. So, I had the dragon bust into the chamber in the middle of the combat when there were only a few of his henchman left.
Lets just say the PCs didn't really know what to think. There was some serious

going on! The fact that they might actually be facing a losing battle seemed to seriously upset a few players.

Retreat? 4e? Naaawwwwww.
When the dragon made his appearance he smashed his way through a blocked-off tunnel on the far side of the map, where the wizard was currently slinging spells at the few surviving monsters. When the white landed next to him, his Dragon's Fury (claw/claw/bite if both claws hit) dropped the wiz and nearly killed him - 3d10 + 18 + 2d12 cold is evil.
The warlord rushed over and got the wizard breathing again, but the dragon covered them both with his breath weapon - the warlord got smacked but I missed the wiz with a natural 1! In anger the dragon spent his first AP and did Dragon's Fury again - the wizard deflected one claw with his Shield spell, avoiding the painful bite and, subsequently, unconsciousness yet again! There was much cheering.
The dragon flew back at this point and hovered above the PCs, blasting with another breath weapon. The rogue was leaping off pillars of ice with acrobatic stunts to get melee attacks off while the paladin used Radiant Charge to fly up and smack him. Eventually the dragon came down to dish out more melee pain and the PCs opened up with every daily they had, dazing him, blinding him, dazing him again, knocking him unconscious (rogue daily Knockout), coup-de-gracing while he was unconscious, surrounding him with a wall of fire, etc. They even had to dip into their precious stock of potions of vitality just to keep everyone going!
When all was said and done the dragon was only 40 hit points or so from death when he managed to fly out of the chamber, cursing the PCs and vowing revenge! High-fives were exchanged by all, and

turned to

when the PCs realized that, with the XP from routing the dragon, they had hit 14th lvl.
In Attack of the White Dragon 2: This Time It's Personal, I won't play the dragon so boldly and will hover out of melee reach to keep the pain going longer. It was already quite late though so I didn't want to drag the fight out too long (with the previous monsters & then the dragon showing up the battle lasted 14 rounds). Plus, I won't have him barge in on an ongoing encounter, so fresh 14th lvl PCs should have an easier time of it.
tl;dr - Successful dragon encounter was successful.