In the Node Editor (Blender 2.67) there is pin icon in the menubar below, which is described as “Use the pinned node tree”. What is it used for?
The pinned node tree feature enables you to stick a node tree (pin it) to one instance of the node editor, so that you can open another node editor displaying another node tree.
This feature is really useful when you need to simultaneously compare and edit Node Trees belonging to different data blocks (materials for instance).
Without pinning the node editor will always pick a node tree based on the selected object/material. Pinning disables this context switching so you can keep working on a shader or node group while selecting different objects.