I just did some research and the USB keyboard indeed seems to use a T400s keyboard so swapping it for an NMB should be doable.
Here's my picture showing the back of the keyboard built into my USB keyboard:http://picasaweb.google.com/lh/photo/-b ... directlink
I found this set of pictures showing the insides of a T400s:http://www.thinkpads.com/2009/07/10/thi ... upgrading/
Specifically interesting is the photo showing the back of a T400s keyboard:http://www.thinkpads.com/wp-content/gal ... d-back.jpg
The only difference seems to be the lack of the bottom screw thread but take a look at this picture of the USB keybord base:http://picasaweb.google.com/lh/photo/E1 ... directlink
The deeper spot in the center seems to be there exactly for the purpose of accommodating the screw thread.
The circuit board is screwed to the keyboard using the thread of the other screw. This thread can't be seen in my photos but I have unscrewed the board and it's exactly the same as seen in T400s keyboard.
The black patch is glued to the keyboard using some double-sided adhesive tape. The tape is only around the patch, not on it's entire surface.
The volume/ThinkVantage/power buttons cover on a T400s keyboard most certainly is removable as it always was on previous generations keyboards.
The only other difference I spotted now is the ThinkLight icon on the page up key of T400s keyboard. The USB keyboard doesn't have that and Fn+PgUp doesn't do anything.
The keyboard from that USB keyboard is totally different from what I have in my T400. I'm sorry. Judging from that picture it seems that the type of keyboard you would need is one that is specifically one made for that model of USB keyboard. I wouldn't know if they made an NMB for it either. Sorry.
T400s, not T400. These are totally different machines.
-- ADDED --
After using the keyboard for a couple of days I can say I like it but it's not very stiff. The T400s keyboard has these holes on the underside which are compensated in an actual T400s by rigidness of the laptop. Here we just have a plastic base that flexes.
Fortunately I was able to fix this by reinforcing the base with a couple of metal rods.
Next time I have it open I'll take a picture.