I have a Nano keyboard, and have tried various MacGyver-esque permutations : inside a modified ring binder, inside a modified CD sleeve case, and whatnot. As Karel pointed out, none of those semi-permanent arrangements worked out, because they were too bulky and too laptop-like, not mobile tablets anymore. And not fluid enough to swap in and out of, because of cables and batteries etc.

Actually I do know what would be the ideal solution (for me). There just isn't anyone to build it anymore.

What I would like is the keyboard part of a Psion 5mx (the 1997 mother of all clamshells, still unequaled, just like the Newton in its class), with just an AAA cell and a BT interface. The screen would be replaced by a hollow cover, that would act as a snug holder for the tablet when open - but keeping, of course, the bit of genius of the (patented) slider hinge.

In short, something like the current BT keyboards, only done right. Past Psion5 users will know what I mean :-)

Nokia already has the Symbian (ex-EPOC32) license ; maybe they could license that keyboard design too...