How do I copy a link to a Mail.app message without using drag-and-drop?

Based on splattne's response, and this macosxhints entry, I figured out how to do it. Now, it's just a matter of using something like Quicksilver of FastScripts to bind it to a keyboard shortcut.

tell application "Mail"
  set selectedMessages to selection
  set theMessage to item 1 of selectedMessages  
  set messageid to message id of theMessage
    -- Make URL (must use URL-encoded values for "<" and ">")
  set urlText to "message://" & "%3c" & messageid & "%3e"  
  set the clipboard to (urlText)     
end tell

If I correctly interpret your question, you could use a variation of the AppleScript in these blog posts:

• Getting a Hyperlink of the Last Sent Message from Mail.app using Applescript
• Speedy creation of rich text links to Mail messages