I dislike the idea of auto-switching to mobile themes in general. I want the user to have the choice considering the fact that smartphones have now more power, bigger screens are wi-fi capable (no 3G costs).
Sometimes though, it is a good practice. Especially in the case of arstechnica.com, which has a design-perfect mobile website. If you are developing websites using Drupal, there are some good modules: Mobile Tools and Browscap.
For simpler constructions like photodiary.gr you can code simple and clean.