visit www.treocentral.com and look under the 700wx forum. the threaded message app works great (except for the keyboard covers the text entry in threaded mode).
Easy work around is to create a new message to the person and it will automatically post it in the correct thread.
Yes, I am currently using it. Works flawlessly except for the fact that it won't show what you are typing except in a new thread window (luckily it puts those into the correct existing threads if you write to someone that you have a thread started with).