Yeah, that doesn't quite do what you want. Here's an example that works better:
Code:
tell application "OmniGraffle Professional 5"
set allGroups to every graphic of (canvas of front window) whose class is group
repeat with _group in allGroups
repeat with _graphic in graphics of _group
set draws shadow of _graphic to false -- turn off shadow for grouped objects
end repeat
end repeat
end tell
Some of these things are challenging to figure out without using something like the Explorer in Script Debugger...