Just to confirm the above statement; it's true that a courtesan's obi would be tied in the front, as stated above, to get it off and on faster than if it was tied in the back. On the other hand, a Geisha ties her obi in the back. Read an entire novel on this.
These guys are correct - studying Japanese and Japanese culture, the courtesans tied their obi's at the front so they didn't damage the incredible elaborate knots, or have to retie them over and over! (coz those are INSANE, they take forever to do....)