I believe the typing speed that can be achieved will plateau at some point, especially when we are talking about the QWERTY keyboard. I have reason to believe that my typing won't get a lot faster than how I am typing now since I don't seem to be picking up a lot more speed beyond 60wpm. Of course, this is because I don't really practise writing English sentences besides this blog. Arguably, my typing speed is actually higher when it comes to day to day MSN conversation, but that's another matter. I remember once delving into how to break a plateau once I do come to it, and I found out about the Dvorak keyboard layout.
My typing speed seems to plateau around this level of 60wpm because I think this is about the speed I can maintain a steady accurate flow of typing without too much backspacing and stopping to let my hands rest. :P
Of course, it could actually be a lot higher than I have tested for. Needless to say, it can be quite amusing to look at your fingers move around the keyboard like mad as you attempt to keep up your typing to some crazy level. :D
Of course, typing speed will plateau. Of course, where it plateaus probably depends heavily on which keyboard layout you use. I'm quite sure your speed will reach far higher on Dvorak than it ever could on QWERTY. :P