I found a workaround - perhaps helpful for some of you - in a blog by Microsoft's Andy Kung @
Blog updated on 3/17/2011. Depending on your experience and perspective, the 'workaround' may strike you as a kludge or a clever solution; it takes a little more work. I think it's clever as I certainly enjoy the RAD provided by LS and don't mind a little extra work.
However, I think pg.203 of Mr. Novak's very good LS book is not entirely true. After Step 5, as reads: "...the Designers in the LightSwitch IDE provide you with everything
you need to create and handle tables and relationships." Having read Chap 7, with a fine!! explanation of the most common relationships, you may be surprised to find that LS 1.0 doesn't support "many-to-many" relationships
in the "Add New Relationship" dialog; the dialog simply says "not supported." Hopefully it will be supported in later releases of LS.