A friend and I were thinking about picking this up, but I haven't really gotten a handle on how the co-op stuff works.  I know that you can get items and maps to dungeons just by sharing data, or something like that, but I want to know what the actual multiplayer questing is like.  Anyone got the goods on this?

Whoever is the "host" invites the other person to their world.  You have to be near each other in real life for co-op to work, it's not like XBLA or anything.  ANyways, you invite them and they show up in your world and replace one of your guys on your group.  You now go out and play the game like you would normally.  They can hang out with your or go out on their own in your world.  You recall them when you need them.  They keep the items and exp they collect but they do not progress in their story.  They are in your game and theirs is ignored.  So, if you advance the story and they see things they haven't already completed, they will have to do it again when they get back to their DS. 
 
It's a great game for co-op on the DS though.

Thanks.  I think that actually answered all of my questions.