I have this odd problem and wonder if anyone else is experiencing this with 3.0.1.
Within Blocks I got to the Default location and set a class of true_header to TOP, true_main to CENTRAL and true_footer to BOTTOM (my creativity has no bounds).
I clear the cache and look at the source of the homepage and true_header and true_footer has been applied. On the other hand, true_main has not been applied.
EDIT: Now I'm completely bamboozled. After I added true_main to the homepage location and true_main to the Products location, true_main is applied but only to the homepage!
If I cannot get it to work how else could i target the 960px of the body?
If I've not been clear, please do not hesitate to ask me for more information.