The AppleScript property selection
for the Mail application appears to ignore whether a single message from a conversation is highlighted in the preview pane (the rightmost pane in OS X Lion's layout). The selection
is determined solely by which messages are selected in the message list (the middle pane). If you want to utilize selection
in your AppleScript for a single message of a conversation, you'll have to select the single message from the message list,
instead of the preview pane.