The original idea for the Eye-Fi series of memory cards was to combine a normal sized smart card with a wi-fi device so that your pictures could automatically be uploaded from your camera to flickr, facebook, picasa, myspace, snapfish, photobucket, mobile me, etc. You could be taking pictures at a conference or a party and people half a world away could be watching the photos show up with no need to connect the camera to a computer. All you need is an Eye-Fi card and a wi-fi hotspot. I own an Eye-Fi card and I still think the technology is magic.
The latest revision of the product is the Eye-Fi Pro X2 which in addition to having 8Gb of memory adds a new trick. The card supports an “endless memory mode”. You can optionally setup the card so that when it fills up it deletes some of the oldest pictures on the card that it has already uploaded to the internet. Imagine being on a round the world trip and never filling up your memory card. Of course you clearly need to trust Eye-Fi that your precious photographs are being backed up but you should be able to verify that using any web browser and a quick trip to your photo site.
Any of the cards in the series can provide a backup solution for your photos but the Pro X2 might also negate the need to take a separate backup device like a laptop.