Fortunately, Rob Griffiths over at Macworld has discovered a simple way around this: hold the option key when hitting Reply, Reply All or Forward, and the original message will stay put while a new, separate composition window opens. This makes it a little easier to refer back to the original message, especially if it's lengthy.
This works on Tiger's Mail.app, but I don't have any older versions of Mac OS X to verify this on. Feel free to sound off in the comments if you've tested this on Panther or even Jaguar. My one complaint about this trick: it doesn't seem to work if you use the keyboard shortcut for Reply. Holding option when hitting cmd - r results in that Funk/hault sound, informing me that the shortcut is a no-go. Either way, this should still be a handy trick for you chronic Repliers using Mail.app.
[via Hawk Wings]