China has boarder disputations with almost every single one of its more than a dozen neighbors. There are two reasons: 1) As a single center force in the region, in tradition China does not have an idea on legal boarders. The world is comprised of two parts, those China care to rule directly, and those that China doesn't bother to rule directly. Even the latter one can only survive when they recognize China's dominance. There's no point to draw a specific line from China's point of view. 2) The last Chinese empire collapsed inwards so rapidly that the actually controlled line of China fell far back to whatever legal or pseudo legal agreements that may have existed. Although in theory China still claims a big territory, but the actual controlled land is far less smaller.
A big part of Tibet, almost all of the agricultural land of Tibet, are now controlled by India. Around 2003-2005, China and India made an deal that Indian would keep quiet about the legal status of Indian controlled Chinese land (so that it may be challenged in the future), while China would recognize India's control over Sikkim. Sikkim used to be a sovereign kingdom under Chinese protection. India took over the place in 1975 with military force, and made it part of India. However, China did not recognize India's ruling until 2005. A document showed Chinese government instructed map be redrawn.
Rumor on the Internet had it that China was moving troops to the disputed boarder, a gesture that China was not happy after India tried to legalized the actual controlling of this land. Chinese are feeling being cheated by the Indian.
In an unrelated note, for the first time in history and finally, the UK explicitly recognised China's sovereignty over Tibet. Foreign Secretary David Miliband issued a statement, 'Like every other EU member state, and the United States, we regard Tibet as part of the People's Republic of China.' Before this, the British has been the only major country in the world that hadn't make a clear acknowledgement of the Chinese ruling in Tibet, but rather resort the Sino-Tibetan relationship to an obscured word 'sovereignty', which was a reflection of the British's view of ever expanding of the kingdom in the 1900s.