Time to stray the topic a bit... Tom, how's your Google-fu as it related to looking up obscure AppleScript info?
I'm finding that a number of mail scripts I've been using forever no longer work in Lion because a certain mail AppleScript property pops an error - even though according to its dictionary it's still supposed to be valid.
This script is supposed to set the selected emails to REPLIED TO. I have another one which sets UN-REPLIED and it fails the same way. Give it a shot on your system and at least I'll be able to tell if it's some problem specific to my setup.
tell application "Mail"
set theSelection to the selection
repeat with i from 1 to the number of items in theSelection
set the was replied to of (item i of the theSelection) to true
end repeat
end tell
The "was replied to" generates the error - it comes back saying it can't set it to true in this case, or false in the opposite script.