Steps to repro:

1. Using JBDS 2.0.0.GA and jbpm 3.3.0.GA, create a new Process Project.
2. Open /<project>/src/main/jpdl/simple/processdefinition.xml
3. Add a new State node.
4. Drag it on top/under the existing (generated) State node. Resize it as necessary until they overlap perfectly.

Compare to JBPM Editor 1.0.2 from SOA Tools project @ Eclipse.org.

1. Using Eclipse 3.4.2 and BPMN Editor 1.0.2 (http://download.eclipse.org/stp/bpmn-modeler/1.0.2/updates/ ), create an empty project.
2. Create a new Bpmn Diagram (under "Other" group).
3. Create some nodes.
4. When you try to drop a node onto another one, the overlapping area turns red to indicate you cannot overlap them. If you release while in this red state, the node returns to its previous location.

In other words, you are prevented from overlapping nodes - only transition lines can overlap.