I've published a sample on how to trigger the touch keyboard in WPF applications when a user clicks into a Textbox, its here:
http://code.msdn.microsoft.com/Enabling-Windows-8-Touch-7fb4e6de
I've been working on this sample for many months and am glad to finally contribute it to our community. Please let me know if there are any questions, suggestions, problems, etc in the sample Q&A pane
Also note that while demonstrate how to "show" the keyboard, i'm not sure how to dismiss it programmatically at this point so its important to test any actual implementation to ensure its a good experience. For example some controls like Button will dismiss the keyboard when they get focus from a TextBox, but certain ones like Checkbox will not and I demonstrate that in my sample.