I just updated to 1.5.3.f1 and immediately found this bug. Doesn't seem to happen when the Playmaker window is floating, only docked.
I can reproduce it 100%. Brand new empty project. Add just the playmaker files. Bring up the editor, dock it somewhere. Add an object and put an FSM on it. Add one state, leave it empty. Run then end. Now left clicking doesn't work in the graph view. Right click still does. Left click still activates buttons and control at the right hand side (FSM, State, Events, Variables), just not inside the graph view.
I'm on Windows 8 64bit. Unity 3.5.7f6