xvkbd is a virtual (graphical) keyboard program for X which provides a facility to enter characters onto other clients software by clicking a keyboard displayed on the screen. It also has facility to send characters specified as the command line option to other client.
The GNOME On-screen Keyboard (GOK) is a dynamic on-screen keyboard for UNIX and UNIX-like operating systems. It features Direct Selection, Dwell Selection, Automatic Scanning and Inverse Scanning access methods and includes word completion.
Some Tablet PCs are equipped with a remote keyboard. Data between keyboard and Tablet PC may be interchanged via InfraRed, BlueTooth or other means. If these solutions are hardware based only, they should work easily with Linux. Otherwise you probably need the technical specifications from the manufacturer.