Keep a mail message open when replying

This simple hint came across macosxhints today and I thought it was too obvious and handy to not pass along. If you're viewing messages in their own window in Mail and would like to leave that message open when you reply to it, simply hold the option key when you click the reply button to open a new window for your reply, preserving the original message you were looking at.

Unfortunately, I can't find a similar method if you're using the reply hotkey (cmd + r) as holding cmd + opt + r results in nothing happening. Anyone else find a shortcut for this?