As some folks have pointed out (thanks!), the MOSS UI Tutorial videos are no longer available on ScreenCast.com. I was using a free account at ScreenCast and my bandwidth limit was reached, so they have disabled my content. I'm looking around for alternatives, but in the meantime you can see the screencasts on YouTube.
Part 1: Master Page Overview and Custom Design Examples http://www.youtube.com/watch?v=7nd7D4JwYbM
Part 2: Prepping HTML/CSS Templates for MOSS http://www.youtube.com/watch?v=Nu2MIGg1sfc
Part 3: Prepping the Files in SharePoint Designer http://www.youtube.com/watch?v=7ScHtsoLXtY
Part 4: Base Master Page Code Overview http://www.youtube.com/watch?v=07zT-ab671s
Part 5: Using the Base Master Page to Create a Custom Master Page http://www.youtube.com/watch?v=GrtQduUwAC8
Part 6: Creating a Custom Page Layout http://www.youtube.com/watch?v=CXoxqGgpB8g
Part 7: Assigning the Master Page and CSS File http://www.youtube.com/watch?v=gN70CjPy80Q
Part 8: Creating a New Web Page http://www.youtube.com/watch?v=Ga-ScG4626s
Part 9: Tips for Styling the Navigation Control http://www.youtube.com/watch?v=J_rWfdI8C-o
Part 10: CSS Variations in a MOSS Website http://www.youtube.com/watch?v=iYqYD0f9b0Y