Doing it in CSS with the view to making the box smart, isnt brain surgery but its a long enough process that if you where to sit down and think about it, write about it, explain it and educate someone else as to the process would end up being a pretty complicated issue.

Fortunately someone has actually bothered to do it.

Check out:

CSS – Single Image Fluid/Fixed Rounded Bordered Corners walkthrough.

Remember, once its done its done. I might be doing an edit of this to loose the drop shadow so that the box can be replaced with a gradient or single tone colour background then package up the PSD, CSS and html page together for lucky beggars to download…

Will keep you posted.