Thanks Jim, that helped me track this.
In VS2010 only, if you hover over the connection point for a foreign key link you'll see a move cursor with a dot in the middle. If you click with this cursor visible to start a drag you'll crash when you let go of the mouse button.
We aren't really set up to handle moving connection points in this view (we have no way to persist them right now because apart from the shape positions the whole view is regenerated with the relational model) so I need to figure out what new VS2010 feature is on by default and turn it off.
In the meantime, make sure you don't put down the mouse button when the connection-point-moving cursor is visible. This dies in the window proc and results in a bad crash, as I'm sure you've noticed.