I understand that the 5.5 version of liferay has a mobile theme set up. However, when I access my liferay site via cell-phone/mobile device emulators, I still get the XHTML theme that I set up for normal browsers. Is it possible to serve different themes for different devices? What triggers the use of the mobile theme in LIferay.

check the BrowserSnifferImpl class ... you can replace it with your own implementation in the EXT environment and basically, the is_wap() can be overriden to check for other things ...