I just wanted to update this topic with some new info.
I was able to get a Silver MacNTouch into my Oct2005 PowerBook.
Removing the builtin keyboard required taking to the upper part of the PowerBook off, in order to remove the 10 screws that hold the builtin keyboard in place. I used the directions found at http://www.ifixit.com/Guide/64.4.0.html
(looks like the incorporated the corrections I sent them from going through the steps myself). So, this goes against what FingerWorks had said, in the past, to explain why they had discontinued the MacNTouch. I wouldn't be surprised that it had to do with the amount of work needed to install the Silver MacNTouch compared to the White MacNTouch. Or, more likely, it had to do with the founders/inventors getting acquired by Apple.
Please note that, as I understand it, the Silver MacNTouch I own is a bit different from some of the other ones that had shipped. It has a longer USB cable that allows me to drap the cable the left speaker, as opposed to piping the USB cable through the USB port as the FingerWorks instructions suggest. Also, this Silver MacNTouch came with 3M double-sided sticky pads to hold the keyboard in place, as opposed to being screwed down. I am not sure how else my Silver MacNTouch is different from other Silver MacNTouches that were sold.
I will say that the older White MacNTouch will not fit at all, at least without some major modifications. The USB cable is designed to connect to a USB port in the rear of the Titanium PowerBooks, so it's far too short to reach the USB ports on either side of the Aluminum PowerBooks. Also there are a set of tabs at the bottom of the White MacNTouch that would need to be removed in order to fit the keyboard well.