As an editor for a school publication, I have a folder in Mail set for content that other students submit. Recently I found myself needing to send one email to every submitter (nearly 150 emails in all!), yet I found that I could not simply do a Command-A and hit Reply.
After a bit of AppleScript work, I came up with this script:
set thesenders to {}
set thesenderstext to ""
tell application "Mail"
set themessage to the selection
repeat with i from 1 to the number of items in themessage
set thesender to (the sender of (item i of themessage))
if thesenders does not contain thesender then
set thesenders to thesenders & {thesender}
end if
end repeat
set AppleScript's text item delimiters to ", "
set thesenderstext to thesenders as text
set the clipboard to thesenderstext
end tell
It creates a comma-separated list of the senders of the selected messages, and then sets the clipboard to said list (it also removes duplicates). Select a bunch of messages in Mail, run the script, then paste the clipboard's contents into a new message.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070521122642952