I know there are quite a few groups/people travelling through Tibet this year. This is just a heads up as to new government regulations on the region. At the moment, mixed nationality groups are forbidden. Only groups of a single nationality with a minimum of 5 people can obtain a Tibet travel permit. As this is such a manifestly ridiculous rule, hopefully it will change soon. It applies to ALL
travellers in Tibet, regardless of mode of transport. See this link to the Lonely Planet Thorntree forum for more info. 2012 PERMIT SITUATION UPDATE - page 9 - Lonely Planet travel forum
For those with plans to travel Tibet in the near future, I would check with your agent for more information.