Our group just finished the DMG adventure too, and we found it to be terribly, terribly easy.
Our party:
Dragonborn Fighter
Half-Elf Warlord (Tactical)
Eladrin Rogue
Elf Ranger (Melee)
Human Wizard
Most of our party beat the dragon on initiative, thanks to the Warlord's +2. The Rogue and Ranger went off to one side sticking near the wall and behind a pillar for cover to launch some preliminary (and useless) ranged attacks, while the Fighter marched into the middle of the ice away from everyone else and used his Dragon Breath to mark whitey. The dragon rushed out, used its dragon breath on the biggest clump (the two strikers and the Warlord), but managed to miss the Rogue and Warlord with his breath weapon due to being marked by the Fighter. It used its action point for fearful presence, which hit everyone but the Wizard, who just magic missiled from a distance the whole fight anyway. The dragon rushed up to the side of the room to try to take out the rogue, but rolled a 1 on one of its attacks and thus couldn't use its bite as well. Then the Ranger and Rogue delayed until the Fighter and Warlord moved into position behind the dragon, to take advantage of flanking positions. The whole party failed a round of saves against the -2 to hit, but succeeded on the following round, in which the party really opened up. The Warlord opened with Lead The Attack, netting the party a nice +4 to attack rolls for the rest of the encounter, and dished out some healing with Inspiring Words. Then the rest of the party (except for the wizard, who used sleep in the previous encounter) opened up with their daily powers, and after a couple more rounds the dragon was toast.
The fight would likely have been more difficult if the dragon had managed to keep the fight on the ice, but the Fighter's anticipation and attempt to counter that tactic by placing himself in the middle of the ice right off the bat seemed to pay off. Even if the dragon had fought on the ice, it wouldn't have been that hard for the rest of the party to surround it and bring it down in much the same way.