Sorry, Mark, but you missed the part where bmx269 said "and not lose the comments that are already there."
Your script works for adding the comments, but it doesn't preserve the old comments. Here's my version:
on open the_files
repeat with a_file in the the_files
tell application "Finder"
set old_comment to comment of item a_file
set comment of item a_file to old_comment & " Here are some new comments!"
end tell
end repeat
end open
Hope that helps!
oaf