well I don't know for sure how the BIOS can access the CDROM right away (maybe like DOS, it needs to load one driver into memory to read from most CDROMs), but as far as I know, bootable CD's actually emulate floppy drives when loading.. at least, the bootable Linux CD's I have do something like that.
most likely the BIOS has a set of generic drivers for it's own mini OS (after all part of the word BIOS is input/output) to read the CDROM, it's just a question of what the files on the bootable CD when they are run (remember, they don't have to be DOS, Unix, or Windows files, as long as they're compatible with x86 they'll run).