I dont think this happens automatically in Liferay version 6.0. In 6.1, it is.
6.1 automatically detects the device type and makes it available for liferay code to use. 6.1 also comes with a wurfl-web plugin which is used to setup rules based on the detected device type. This plugin relies on a wurfl repository available at http://wurfl.sourceforge.net/.
In 6.0, if you just want a perform redirection based on a simple rules, then you can use a filter which will check the user-agent and perform the re-direction.
If you want to alter the theme and layout then you can use ServiePreAction as described in http://www.liferay.com/community/forums/-/message_boards/message/11601895
Or you can add full fledged device recognition and rule setup capabilities similar to 6.1 using ext environment.
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