After quickly coding up my own solution, I decided to do a deep search for different modules that do this; here are the results so far:
Admin theme
Blog theme
Dynamic Theme
MobileTheme
Manage
Node style
Role Theme Switcher
Referer Theme
Sections
SwitchTheme
Theme setter
ThemeKey
Taxonomy Theme
View - Theme
User Profile Theme

The Mobile Device Detection module allows a site owner to handle the redirection of his users to his mobile site. This module does not a transformation of your site to a mobile version, but it assist you in handling the traffic to your mobile and desktop site. You can choose the most user friendly way of redirection your Visitors.

This theme is intended to return only clean HTML with no styling (images and styling in content is maintained)

A module wich switches Drupal theme according to user agent, using handsetdetection.com web service.

mobit (MobilizeIt or MobileTheme) is a module that allows users to choose a theme for use with mobile devices. It has 2 parts. Part one allows the administrator to define one or more of the installed themes as themes appropriate for use with mobile devices, meaning not many images, no javascript and a simple layout.