There is a DiagramItem.Id property, but it is intended to be assigned by you and is not set automatically by the control. You could set it on the client side by calling e.getNode().setId() from NodeCreatedScript, and after postback you will be able to access it on the server via node.Id. For example, you could keep an integer counter as value of diagram.Tag, and increment and assign it to new nodes' Id when users draw them.

I hope that helps,
Stoyan