Not sure if this is a bug or not but with the upgrade from Richfaces 3.2.0 to 3.2.1. The rows are no longer truncated where the horizontal scrollbar begins, they are instead stopped at the bottom of the scrollbar. I believe that whole component is called the .dr-sdt-inlinebox.
This only occurs in IE6/7 and once you scroll around in the data table to make it refresh itself the problem disappears and the rows render underneath the horizontal scroll bar as normal.
Actually turned out to be the columns widths causing the issue.