Based on your description of the dragon, I would think that he (or she) would have an incredible curiosity about the world. It is probably a world that may see this curious dragon as a frightful menace. This lack of understanding of the world would lead to many of the interesting consequences that have already been mentioned.
I would think that this ignorant, out of date dragon would be fairly useless in combat or be just as much danger to the party as their foes. The dragon would also probably be too big to go most of the places that he party goes.
Interesting events could occur when this dragon meets other modern dragons of your world, particularly evil dragons.
The dragon may decide that he (or she) likes (or loves) one of the characters more than the others. Maybe this is the character that first offered food or contributed the most to the skill challenge. This dragon, particularly if it has an evil nature, may start scheming to isolate the favored character or do away with the others.
In the end, I would think that the dragon should become (at best) an NPC noncombat ally or patron that the party can occasionally turn to for some assistance or information rather than a combat pet. Maybe the dragon can fly the party somewhere in times of great need. The dragon may have artifacts or ancient books as part of its treasure horde that the characters can borrow or use for research. The dragon could represent the party as a diplomat to other dragons on occasion possibly.
I understand that you do not wish to punish the characters for successfully completing a skill challenge rather than solving the problem by combat. However, I do not see how you could allow a creature that is probably as powerful as all the characters put together to regularly assist in combat without ruining your game. The only exception that I can think of right off hand would be background or storytelling combat. For example, the dragon flies several passes breathing lightning on the front ramparts of the Frost Giant castle to distract the defenders while the party sneaks in a hidden tunnel in the back. This way the dragon is assisting the characters in their war, but not fighting thier battles for them.
Have fun with your dragon!
