There is currently no registry key available to change that setting. I am also not aware of any plugins that change the behavior. However, I do know of two methods of creating a new text message from the Today screen:
You can either set the left soft-key as "New SMS" using
this registry tweak that I created, or you can install the
9-icon HTC Homeplugin update that will put a few new icons in the usual HTC Homeplugin tray on the Today page.