You’re probably just looking for the width() function..
See the docs, it excludes the padding: http://api.jquery.com/width/
(Just as intended in modern browser’s representation of the css width property)
Edit: It’s now 2012 and jQuery 1.8 is just coming out. While this is still relevant, you may also want to read the following article from the jQuery blog regarding box-sizing considerations in version 1.8