I'm considering a DS + AC DS purchase.
But AC DS would have to truly offer something new and not be Animal Crossing v1.5
The original was for the N64 and Nintendo has since whored out the code and added and tweaked stuff and released it several more times. There's the N64 version > GC jp version > GC NA version (with password trading) > GC jp v2 version which is the NA version ported back
All of it was practically the same game with a few items added and a few things moved and changed around. A DS AC in the vein would be relatively uninspired and boring.
Can you imagine how sweet Animal Crossing Online would be?
I'm not convinced AC would work well as an MMO type game where there's a persistent universe and people could come and go etc. There's too much room here for malice, and the game, like all MMO would quickly evolve into a rat race of who got the most bells, most furniture and it takes away from the liesurely playground the AC offers.
The AC games tend to emphasize co-operation and cohabitation over the traditional MMO character building race that is pretty much standard in any MMORPG today.
Besides, I'm highly skeptical Nintendo would start an on-line service and invest that much money into an AC game that consumers might have to end up paying a monthly fee to. If they do, it'd be a radical turn for them.
What I'm expecting, assuming it is on-line at all, is a battle.net type free service with a relatively cheap infrastructure. That means there's probably a trading house system on a Nintendo server AC users could log into to trade items and download and upload little bits of stuff. Their towns might be allowed to be uploaded as a kind of scale model that people can visit and play in, but ultimately, much of the playing will occur offline, with the on-line aspect being secondary.