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If you're using OmniGraffle 5, there's a checkbox in the Canvas: Size Inspector to "Print canvas on one printer sheet", if you're using version 4 or version 3, there's a "Fit" button instead to automatically set the print scale to a single page (you'll have to manually set the canvas size to one page in this case first).

Hope that helps,
"Vroom! Vroom!!"