I speak from experience when I say that web developers should understand the basics of cascading style sheets (CSS). Granted, when I first looked at CSS and tried using it for page layouts a few years ago, I couldn't understand why anyone would use CSS for layouts when we all used tables instead*.
Well, years later I'm convinced that tables are for presenting tabular data, not page layouts.
Developers don't have to be CSS gurus, but they should understand what a "div" really is and how a "float" style attribute works. They need to understand the "boxing model" and how IE handles it differently than do other browsers. CSS is much more than just applying different colors and text attributes.
If you want to take your web development skills up a notch, I recommend the following book I purchased a few years back. It is short, concise, and easy to read. Plus, as far as books go, it's pretty cheap.
CSS Mastery from Amazon
Update: A major advantage to knowing CSS is that it will make the adoption of tools such as jquery much easier!
*About the only time I favor a table layout is when I have to bottom-align tabular data and don't want to set a fixed height for content.
