Ellen Anon writes in her recent post, Memory Card Failures, that "Sooner or later it happens to almost all digital photographers - at least all those who shoot a lot. You check some of the images in camera and they look good. You load the memory card into the reader and wait for the images to appear in Aperture’s import window. But nothing happens. You push the card into the reader a little more firmly and make sure the reader is attached properly to the computer, but still there’s no sign of the card showing up on the computer."
"The first time it happens it’s hard not to panic, especially if the card contains shots that will be difficult to replicate. Plus you wonder if the card is corrupt and should be replaced, or if you can salvage it. And you wonder if it just happened out of the blue or if you somehow contributed to the failure."
What follows is some good information about handling memory cards and what to do when things go wrong. Also, check out the discussion at the bottom of the post. Some great comments by knowledgeable people round out this helpful article.