This is not probably correct. This is just a attempt to understand and
make a solution to the problem. A bit like discussion initiator.
(source of information, English and Finnish wikipedia)

History:
========

21 BC:       Tibetan Plateau inhabited.
2 BC:        Neolithic immigrants from northern China replaced the
	     current population.

700/800:     Tibet big power in the area (even emperor of china). 
800-1500:    Area ruled by Mongolians every now and then.
1500:        Teokratia: a country ruled by religious people.
	     In power: lama Sonam Gyatso (1st dalai lama).
1700:        Military victories by the powerful Fifth Dalai Lama 
	     consolidated Tibetian power over Ü-Tsung.
1800:        British empire and Russian emprire expanding.
             China claimed Tibet as a protectorate (note, not
             as a a part of China). 
1850:        Presence of all foreigners banned. Tibetan who 
             unwittingly gave assistance to a foreigner, as the 
             government believed it would affect the fate of the 
             country and destroy its culture and religion.
1906:        UK and China make a deal to suppress Russian power.
	     => Tiibet was assigned for China.
1910-1912:   China occupies Tibet for first time.
1912-1950:   China crashed, Tibet was independent (although none of
	     the other countries acknowledged it to be independent).
1950:        Army of China marches to Tibet to "liberate Taiwan and Tibet".
	     40k chinese army against 7k Tibet army. Enforced "peace treaty" 
	     signed and threaneded Dalai Lama escapes.
1959:        Rioting/restlesness increases, chinese army silences the
	     riots with bloody consequences. Panchen-lama as puppet
	     master in Lhasa.
1965:        Tibet gains "autonomy", which meant buddish education system
	     to chinese, properties socialised, people put to work, 
             Tibetian cultural items destroyed (including most monesteries),
	     monks enspironed and executed. From this day having a picture of
	     Dalai Lama meant enprisonment. China has utilised the areas
	     resources heavily which has caused e.g. erosion.
1979:        Some religious freedoms were returned. Many han-chinese moving
	     in the area, especially to Lhasa and other big cities. 
2005:        Railway connection between Qinghain and Tibet. Might increase
	     moving in of Chinese.

Analysis:
=========

Tibet has been clearly de-facto independent until 1910. Since that
time they have suffered from the chinese invasion by losing their
land, language, religion and culture, which cannot be anyway
justified. If the purpose of China was to liberate Tibet, it should
have withdrawn from Tibet and give the Tibet freedom to choose its
leaders. Tibet was not probably very nice country under the Dalai Lama 
but it is definitely not better at the moment.

The power of Russia has lately decreased significantly and the power
of China increased significantly so no justifications can be argued
based on security. Tibet itself has not been threat in any point.

Fair solution: 
==============

China should withdraw from Tibet and the han-Chinese who have moved to 
the area since 1950. China should pay the restoration of the area and
natural resouces. This would be at least 50 000 million dollars, which
is the estimated profit they gained from wood industry alone.

Realistic solution: (possible UN treaty)
===================

China withdraws it troops from Tibet. The people living in Tibet
(Tibetians and new immigrated Chinese) will have elections to choose
form of government and the leaders. The elections have to be monitored
and found fair by external and independent observers.
 
APPENDIXES:
===========

WHO WERE LIVING IN THE AREA OF TIBET (area of http://en.wikipedia.org/wiki/%C3%9C-Tsang):

prior 1903:       http://en.wikipedia.org/wiki/%C3%9C-Tsang
1903:             Tibet (100%) source: After British_expedition_to_Tibet
2000:             Tibet (94%), Han-Chinese (6%) 

Note: In 2010, a study of genomic variation suggests that the majority
of the Tibetan gene pool may have diverged from the Han around 3,000
years ago.