I normally just take the card that is in the camera. No backups.
I also haven't been on a vacation longer than a couple of days since we moved into our new house in 2002. (Buying a bigger house in a nicer area kind of took away our vacation budget

) My trip this month will be the first time I do not take a film camera also, but I am taking two digital cameras.
I will have three cards total with me -- the 1GB in my 8.1 MP Coolpix 8700, the 256 MB in my Powershot S10, and either a 128 MB or a 64 MB backup in my purse.
Conveniently, my Nikon, my Canon, and my mom's Canon (she will be with us) all use CF cards. My mom is not computer savvy, so I generally swap the 128 MB and 64 MB cards with her, download her pics, and then swap them back when her card gets full again.
I would like to get a 512MB for backup, but the one time I filled up my 1GB, it had over 300 pictures in FINE mode (second only to RAW in data size), and I can't imagine taking that many pictures in a day, even at WDW. (We will have our laptop with us -- I will be able to empty the card as needed.) I just always have this worry about cards failing, even though I have had that 64 MB since I bought my first digital camera in 1999.
--Katie