As a long-time computer user, troubleshooter, and builder, I live by the motto, "BACK IT UP - NOW!"
Although I think the CD burning service in the parks is way overpriced, as are so many things at WDW, I do think that burning a CD every one or two days is absolutely nessecary if you don't have some other device with you like a laptop or portable CD burner.
Memory cards are a fairly reliable technology, but like any technology, the more you ask of it, the more prone to failure it becomes. Shooting hundreds of pics at WDW is a trial by fire for most memory cards.