See here for Linux on Tablet PCs aka WebPads.
This model was manufactured in 1994 and has a detachable screen. It features also a pen input device. You may find further information about Linux on these machines at:
This machines made by Fujitsu-Siemens(http://www.fujitsu-siemens.de/) has a detachable keyboard. It's connected to the main machine via BlueTooth port. I couldn't check it's Linux capabilities yet.
This machine has a detachable keyboard. It's connected via an IrDA port. I couldn't check it's Linux capabilities yet. You may find further information about Linux on this and another Scenic Mobile model at:
Sorry but I just have the idea to shape such a machine by mself. But currently I don't have a solution. There are foldable keyboards (e.g. the Stowaway(tm) from Targus(http://www.targus.com/) ) and mini keyboards (e.g. the HappyHacker(TM) from PFU America Inc.(http://www.pfuca.com/)) available.