I think porting old DOS games to modern mobile devices would be very difficult. It would probably be easier to develop a new game from the ground up, although they could always remake one of these games or make a game in the style of Arena or Daggerfall.
I think creating a game with the same look and feel would certainly be possible. Not sure about the size and scope though. Definitely worth playing though. Ravensword did this on the iPhone, and although not a full RPG it did capture an Elder Scrolls "look". The market's fairly crowded on the App store. Perhaps the returns are not worth the risk? I think a small team of 3-4 people could create a very nice small game in less than 6 months. Small investment, but chance of big payback, and also a presence on that platform.
The same argument could be used for DS too.
Mark
(Wilty TES artist)
www.mjonesgraphics.com