xvkbd is an option... http://packages.ubuntu.com/dapper/xvkbd
xvkbd is a virtual (graphical) keyboard program for X Window System
which provides facility to enter characters onto other clients
(softwares) by clicking on a keyboard displayed on the screen. This
may be used for systems without a hardware keyboard such as kiosk
terminals or handheld devices. This program also has facility to send
characters specified as the command line option to another client.
Illustration is from the net.
The same as matchbox-keyboard-im
Matchbox-keyboard is an on-screen 'virtual' or 'software' keyboard,
designed for touch-screen devices running X.
Matchbox is a base environment for the X Window System running on
non-desktop embedded platforms such as handhelds, set-top boxes,
kiosks and anything else for which screen space, input mechanisms or
system resources are limited.
This package provides a GTK+ input module for use with
Matchbox-keyboard or other on-screen keyboards.
Illustration is from the net
Both they are in my repositories on Natty but further information can be reached on the official websites.
I remember to have used one of these when installed Jaunty on a Touch Screen HP Laptop, and can inform that it works pretty fine.
I hope this help.