I can't seem to find it any more, either. I hope it's alright to do this, but here's the script as I copied it a long time ago. Feel free to delete this if anyone things it's not alright for me to publish someone else's work.
Code:
tell application "OmniFocus"
tell content of document window 1 of default document
set taskList to get value of selected trees
repeat with aTask in taskList
set theProject to containing project of aTask
move aTask to beginning of tasks of theProject
--move aTask to end of tasks of theProject
end repeat
end tell
end tell