Just putting in my two cents...
It's always a good idea to have extra memory cards. You never know when you'll want to take extra pictures or movies (if your camera has that capability), and it's better to have too much storage than not enough.
As the resident tech nerd in my family, I've become the family "expert" on digital cameras, and I've fielded many questions.
If you are interested in backing up your pictures so you know you have them when you get home no matter what, I would suggest using the Kodak service at WDW. Yes, with certain deals, you can get memory cards cheap. Get them anyway if you want extra storage space, but still back up your pictures if it will make you feel better. It ends up being the price of a special souvenir, and what's more special than your pictures (although heed the warning of the folks whose CD was blank. Don't leave the store without having them reinsert the CD and verify the files are there).
Memory cards are a somewhat volatile technology. Of course, I use them every day, but that does not negate the fact that they are often unpredictable. I would never trust a backup memory card to truly be a backup for my primary card. When backing up files, it's important to utilize a different technology to do so (I also use my iPod, as one poster suggested). You're exposing yourself to the same primary dangers if you rely on another memory card as your only source of backup.
Hope that long-winded answer helps! I would definitely recommend backing up to a CD during your trip...peace of mind wins out!
