I have started playing with device detection when coding Liferay apps and I have found very useful the Device API in Liferay 6.1.
This API allows you to detect the capabilities of the device which is accessing the portal (the size, operating system, if it has touch screen, camera, compass... etc) and then you make your apps behave different depending on the device. For example, you can add some behaviour only for iphones, or androids, only for small screens...
I have just started a wiki page showing some examples on how to use this API, feel free to add more to this wiki page: http://www.liferay.com/community/wiki/-/wiki/Main/Device+detection+capabilities
There are many more things, specifically for the front end related with mobile development, but I will talk about those in another blog entry.
See you soon!