Thanks, I will try this.
If I ever have to add another class, your function may not be very good as it can only add one class, if I understand well (BTW, the second code the first "top" should be replaced by "bottom"... typo). So unless I use the classes already available on WP 2.7 (as stated by cdharrison) it is my understanding that I will be limited to those and only one more if I use your hack.