Woohoo! This bookshelf I liked is the Offi Bench Box (with tube legs, not casters). You can get it with a pad too.
I had decided I was just going to get Ikea's Expedit Bookcase, turn it on its side, and find steel tube legs - now I'm not sure. The Offi bench isn't available until 2009 though, and my son needs shelves now.