Hi there...
I have the same problem. I have a Canon PowerShot SD880 IS. I'm using a ScanDisk SDHC 8BG card. When I plug in the camera and open iTunes, it's not recognized. I also have a card reader, but same thing - no joy. So I tested both the camera and my card reader with the memory card that came with the camera, a Canon SDC-32M... Works perfectly. When I plug the Canon card into the card reader, it shows up on the desk top instantly, and when it's in the camera, iPhoto recognizes the camera.
So I guess it's pretty obvious that the ScanDisk memory Card is shot, but just to be thorough, I tested both cards in my HP C6180 Printer. The HP recognizes the Canon card, but says there's a fault on the ScanDisk card - Bummer, I just came back from vacation and have 372 photos on that card, (which still works fine in the camera by the way - the camera is still able to record to the card).
Also, just FYI, I have been using my camera with this ScanDisk card and iTunes for months with now. I guess it just failed????