Working here on 10.8 with OmniGraffle Professional
Minor edit to ensure that a valid zoom value has been entered:
Code:
tell application id "OGfl"
tell front window
set currentZoom to zoom
set theDialog to display dialog "Zoom all canvases to:" default answer 100 * currentZoom
try
set theZoom to the (text returned of theDialog) as number
on error
display alert "Enter a number, without a percent character"
return
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