What will be the impact on China of the fact that the USA has begun a relation with Myanmar?
Of course, we can not be sure as to what impact this (and also things like the US decision to station troops in Darwin, Australia and to join the East Asia Summit) will have on China. There seem to be two extreme possibilities and any range of other possibilities in between.
It could be that china will feel encircled and threatened. This could lead it to act even more aggressively towards the US and towards the countries of Southeast Asia. In the worst case scenario, this could lead to war.
It could be that China will come to understand that it must coexist peacefully with the US in Southeast Asia. It might realize that its rise scares other countries in the area to the extent that they will turn to the US to balance China. If China realizes this, they might start to be less aggressive in their relations with these countries . They might stop pushing so hard, for example, on their claims to things like the Spratly Islands.
My own prediction is that their reaction will be closer to the second of these options than to the first. Particularly once China's transition to a new leader is completed in 2012, I think that China will move towards a more friendly attitude towards its neighbors. It surely realizes that it will have tremendous economic and diplomatic power in the area and that it will not need to be foolishly aggressive in order to increase its regional power.