After an update to Ice Cream Sandwich, xperia phones (xperia pro, xperia arc, walkman and others) no longer automatically mount through USB in linux. In windows they offer to install PC companion.
Some are puzzled because the option seems to have vanished from the settings. The option is still there but was apparently moved and is no longer active by default. Disconnect the USB cable and go to:
Settings > Xperia™ > Connectivity > USB connection mode
Change between the modes and connect it to the computer or device where you want your phone data mounted.