'Regrettably, iPhoto no longer offers an option to back up your iPhoto library to optical media. Rather, you can burn only images to the disc rather than the library itself and you can't burn to multiple discs if the selected number of photos won't fit on a single CD or DVD.'
There is a command-line based workaround for that deficiency. You can split the iPhoto Library into DVD sized tar archives and then copy the archives to DVD's.
To create the archives, type the following command into a Terminal shell:
/usr/bin/gnutar -C $HOME/Pictures -L4588544 -cMv --file=$HOME/Desktop/tar_archive.{tar,tar-{2..50}} iPhoto Library
/usr/bin/gnutar -xMv --file=$HOME/Desktop/tar_archive.{tar,tar-{2..50}} -C $HOME/Desktop
[crarko adds: I haven't tested this one.]

