This tutorial was recorded using fixed-width columns. At first, it appears that the faux column background image only works with fixed width, but it can be used just as easily with fluid (percentage) width columns. You have to make sure you have one additional piece of CSS applied to your container DIV:
#container {
background-size: 100% auto;
}
That will ensure the background image resizes at the same rate that your columns do (of course you have to build the image at the same percentage widths as your columns. I recommend working with the largest width it is used at (think CSS max-width setting).