With the standard seat removed, the highest solid object beneath it is revealed to be the flat plate of steel shown here. It sits between the frame rails at a height of 22.5 inches / 570mm off the ground. As can be seen in the action photo elsewhere in this section, it is entirely possible to ride the bike with no seat at all, and actually not that uncomfortable! PNB
See here: http://bikeweb.com/node/3087