Rishi has created a Virtual Input Keyboard and accompanying behaviours, that allows you to attach an on-screen keyboard to any receptive UI control.
It is important to appreciate that the Silverlight Keyboard presented here is an Input Keyboard, which is different from a general purpose Virtual Keyboard found in most Operating Systems. The input keyboard is specifically only made visible on-screen when directly editing/inputting to an attached UI control (e.g. a TextBox), when not editing/inputting it is automatically hidden from view - this is very much like in mobile phones.