You use the message viewer. Something like this in Jaguar:
tell application “Mail”
activate
set the_messages to (every message of in mailbox)
set mess_viewer to first message viewer
set selected mailboxes of mess_viewer to {in mailbox}
repeat with this_mess in the_messages
set selected messages of mess_viewer to (this_mess as list)
delay 1
end repeat
end tell
beep 3