It seems like in the 2.73a version (and 2.74), bpy.context.selected_nodes method is not available despite the documentation showing it up. Does anyone have an idea on what could be happening ?
You might be able to filter on node.select for your application:
mat = bpy.data.materials['checker'] nodes = [ n for n in mat.node_tree.nodes if n.select ]
Although, I'm not entirely sure how that works with node groups (where you have a compound node selected and tab into it and select other nodes).