I think this is a problem with OS X, at least with it's CD burning. Get Toast, it will let you use all of the available space. Since this is your first post, you are probably new the Mac experience. This question would get better answers in the Mac OS X forum (just in case I am wrong). Not that big of a deal, but the iBook forum is usually pretty slow compared to the Mac OS X one.
On a side note, does anyone have any idea when or if we will be able to use a CD-RW like a floppy? I remember hearing that Mac and Windblows were working on it. Is this ever gonna be a reality??