While I'm sure I'm not alone in wishing this could be put on a DS or like things, there are issues that will vastly complicate that.
Firstly, as others have mentioned, is formatting. It would need to be converted into a format that a DS can recognize. I don't know whether Sphere could be recognized on the DS, but, from what I gather from the other posts, I doubt it without much work and many changes.
Secondly, and probably most important, is filesize. A .ds file (the usual format for ds games) has a filesize limit, and thus limits the features that can be put on it (I have been yelled at a few times for my idea on a hack because of this issue). I've come to realize that this is the reason that nintendo does not have more than 4 moves on a pokemon at a time, why they don't put in more than one region at a time, and many other things. Simply, with all the memory they have to have with 493 pokemon, two gender versions, and shinies of both of those, then you add up all the data that comes from creating routes, trainers, etc, everything that comes with creating a region, and you can see how quickly the storage available is eaten up.
Unfortunatelly, I'm not super-familiar with the .ds file format, what the actual size is, or what it can do, but what I do know is that if this game was ported to the ds or other mobile device, a lot of features would have to be nixed...and/or graphics would suffer as well. I hope this answers your question.
