I dont think the games for the 3DS are going to be playable on the DS, simply because I think (and hope) that the 3DS will be getting a bump up to the gamecube graphics. Add that to the interview with Saturo saying that a next-generation handheld would need higher-quality graphics and I'm pretty convinced that that kind of compatibility wont exist (I also dont think Nintendo would be able to justify another release that wouldnt be a new series). Plus, Nintendo's handhelds have never been cross-compatible across series (not counting the GB and the GBC as different series).