Very nice

I developed my own YUI CSS based Drupal theme that I meant to submit but hadn't had time yet. I'm glad that you did because YUI CSS can make a lot of Drupal themer's lives easier :)

I'm going to go through and compare your version to mine. In the mean time, I have a few questions and suggestions.

@Hosted vs. Local YUI CSS
Yahoo!'s hosted YUI files seem to be fairly reliable but had you considered documenting that users can download the YUI CSS files, save them to their theme folder, remove the @import YUI lines from styles.css, and link the local CSS files in page.tpl.php?

@Leveraging Other Drupal YUI Modules
Related to the first item, should this theme be dependent on the YUI module? What about the providing the ability to use the YUI Menumodule for building drop-down menus for primary links?

@Automatically switching left right sidebars
I've added logic to page.tpl.php to switch doc and yui-tN classes based on sidebar settings. This allows sidebars to switch dynamically based on block visibility settings. Seems like this logic could be easily added to phptemplate_page_template().

@Print Style Sheets
Too few Drupal themes provide them and they're so easy to make.

I'd very much like to contribute to this theme.