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Working here on 10.8 with OmniGraffle Professional

Minor edit to ensure that a valid zoom value has been entered:

tell application id "OGfl"
	tell front window
		set currentZoom to zoom
		set theDialog to display dialog "Zoom all canvases to:" default answer 100 * currentZoom
			set theZoom to the (text returned of theDialog) as number
		on error
			display alert "Enter a number, without a percent character"
		end try
		set theDoc to its document
		set currentCanvas to its canvas
		repeat with theCanvas in (every canvas of theDoc) as list
			set its canvas to theCanvas
			set its zoom to theZoom / 100
		end repeat
		set its canvas to currentCanvas
	end tell
end tell