Both have their advantages. Which is best for you depends largely on what you shoot and how you shoot. Two cards wins on security, both hardware/card failures and IO (read: Idiot Operator) error. Most data loss on modern flash memory cards is the result of IO error.
Personally, I vote for two cards. My shooting is generally slow and unhurried and occasionaly covers a long period of time (vacations). Switching cards is not a problem. For the really paranoid, you can switch cards back and forth periodically so that if one card looses data, the remaining pictures on the other card cover the whole job/vacation with just small gaps. No massive omissions. National Geographic used to follow that procedure in the "ancient" film days; they numbered each roll throughout the shoot and processed the even nubmered rolls first, then the odd. That way a lab accident would only reduce the number of images of a particular part of the coverage. It wouldn't wipe out a whole segment.
Imagine you're at something like an airshow and you want to take *more* than 2gb of RAW files. The best thing to have is 2 1gb cards and then add a £100 personal storage device of about 40gb. Thing is, these storage devices are quite slow to transfer over the data. With the 2 card setup, once you fill the 1st card, you put it in the storage device and start copying across, which will take about 10 minutes. Meanwhile, you whack the 2nd 1gb card into the camera and keep shooting. Each time you fill a card, you wipe the other card and put the full one in the storage device.
If you only go with 1 2gb card, once it's full you'd have a 20 minute wait while you transfer the contents onto your 40gb drive before you can continue. How many shots would you miss at an airshow in 20 minutes?
I hope this explanation makes sense!
Also, I agree with the views about lost cards or corrupted or accidentally erased data. If I go on holiday and take 2gb of shots, I'd feel much less bothered if I somehow lost 1 card than if I lost the lot.
I vote for the 2x1Gb card system but instead of buying slow personal storage device buy a fast one. Compact Drive PD70X should be good and the newer version "hyperdrive" or something should be even faster. With Compact Drive PD70X 1Gb Sandisk Ultra 2 SD card filled with 178 NEF files (961Mb) is copied in less than 3 minutes. Autonomy is also very good as PD70X can copy up to 35Gb on a single set of NiMH batteries.
I agree with the air show concept myself as I also shoot air shows. I myself use 3 cards. A 1GB, 256mb and 128mb. I just recently purchased another D70 so I now have 2 cameras to use at airshows. I am also going to purchase another 1gb card, I hate to be frantically copying a full card when the next performer is coming out, it does take a long time. With two cameras this could be a worse problem so in my case I really need three 1GB cards. So I say, the more smaller cards you have, the better off you are, just make sure you don't get them mixed up, or forget which one you have filled up, always format or delete after images are copied to storage device or laptop.