kmac,
Actually more iBooks G3 (Dual USB) came with DVD capability than without, but some are CD-ROM (only reads CDs) and a very few others are CD-RW (reads and write CDs but no DVD capability whatsoever). Some of the early models were DVD (reads both CDs and DVDs), but many have Combo drives (read both CDs and DVDs, and writes CDs).
So, whether or not the iBook has DVD capability depends on which particular model (MHz rating and production date) it is.
I kept my iBook G3 (Dual USB) 800 MHz (with the Combo drive) in Mac OS X 10.3 (Panther) because it is capable of booting with the original 128 MB of RAM which is soldered to the logic board (whereas Mac OS X 10.4 requires 256 MB of RAM). Once in awhile RAM fails, and it is simply my cautious nature to want to be able to troubleshoot RAM problems by removing the 512 MB extra RAM module and still be able to use the iBook (albeit slowly).
However, if you are needing Mac OS X 10.4 and the iBook is DVD-deficient and you have the DVD version of Mac OS 10.4, there is a way around the problem, provided you have access to another Mac with FireWire.
http://www.oreillynet.com/mac/blog/2005/05/thecats_mustachesinstalling.html