mobile site question
Drupal already has a UI layer in the theme. There are modules which will allow you to switch the theme depending on the device (have a look at mobile_theme), so you can have different themes for different devices. This way your modules should only send data to the theme layer which then acts appropriately for the device viewing. 

However with the increasing sophistication of mobile devices it is possible to create a site which has the same markup but adapts to the device using clever css and jquery.