Ok, First off thanks always for the great work you guys do. To make sure this wasn't me -
I did a clean install of the Arduino IDE, added the libraries for the TFT shield, Touchscreen, and SD, and plugged in the UNO with the shield attached. I ran the TFTPaint demo, and voila, it all worked fine.
I'm not sure what happened before, and on two separate development computers, but I suspect it must be another library creating some incompatibility. I had been using other LCD screens so there must be a conflict there.
Everything seems to be working fine.
Thanks again for your help.