a favoured clearing method is giving the element "overflow:hidden;" It TOTALY ELIMINATES the need for having CLEARING ELEMENTS in the markup. (Thanks Paul OB)
As far as rounded corners, well i assume you are going of full IE6 support, otherwise I'd just use CSS3's border-radius property and eliminate ALL markup that is there solely for the purpose of making the corner.
If you are going all retro, I suppose your method is not too bad, but i will submit my standard practice for this.
the only draw back is if you had a fading bg. It looks like a lot of code , but actually it syncs with your regular code easily.
<div class="top"><b></b></div> & <div class="bot"><b></b></div> are simply sliding doors, you can alos ad any top/bottom margin and clears for layout to these classes... further slimming your code overall, after that you can either contain your content in a .box_content div, or merely add the class to the BLOCK element itself . #sidebar was there to indicate the rest of the layout.
hope that helps