I bring a small laptop with two portable external hard drives. I download the photos each day to each of the hard drives for back up. Then I format the sdhc cards for a clean start each day. We have two (mine and my DW's) DSLR's and each have two batteries and two 8gb cards. We've never had a problem with this system. We average about 3000 images per WDW trip. When we get home I go through and delete the ones we don't want and my DW will pp the ones we print. Its a team effort and works well for us.
That's a good plan, redundancy is key. My plan for my upcoming trip (now that I've installed a much larger hard drive in my wife's laptop - I ran out of room last trip!) is download the pics nightly and burn DVDs of the day's pics. Last trip, I was burning a pair of DVDs then having to delete them from the hard drive to free up space for the next day's pictures!
The more places the pictures exist, the safer they are!
...
Deleting pictures in the middle certainly shouldn't make a difference. Your memory card functions like a small hard drive. If it's 2gb or less, it's formatted in FAT format, and if it's larger than 2gb, it's formatted in FAT32. FAT is the same file system used by DOS and Windows 3.x, FAT32 is the same used by Windows 95/98/ME. There's absolutely nothing magical about the way the files are treated on there. As long as you're not removing the card while it's being written to and the camera's software doesn't have file system bugs, you can delete to your heart's content with no fear of losing data. Since the data is stored randomly across the actual memory inside, it's not even like you're putting a hole in the middle of your data that gets filled in later.
...
Costwise, I think it's $13 for Disney to burn a CD for you. That means that, even if there were no limits on how many pics they'd put on a CD for you, you'd be looking at $39 to empty a mere 2gb card! In other words, it's waaaay cheaper just to bring more memory than you can imagine ever needing. Plus - who wants to spend their vacation sitting around waiting for CDs to get burnt?