We're glad you like and you're excited about our new Diagramming control. Printing is one of our biggest challenges that we'll work on this Q, so that it should be available for the upcoming Q2.2012 release (when the official version of the RadDiagram will be released). For the moment the best you can do is to export the diagram to image (PNG) and write some custom code to print the image.
On a side note you can provide real world printing mechanics on your own, but this will involve much more effort on your side. If you decide to go that way, we can provide you with help and support during development.
Hope this helps.
All the best,
the Telerik team
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