If some images import fine but not all and they will not open with Preview, these images are most likely damaged.
See Old Toad's post at the bottom here
http://discussions.apple.com/thread.jspa?threadID=2465859&tstart=0.
He references a script he made called "Convert to JPG and Embed sRGB profile".
Try that script to see if it makes the files readable.
Consider using these best practice tips to avoid or minimize the impact of issues in the future:
_Ensure your camera has a full charge on the battery before you import (Or run off of AC if possible).
_Ensure the date and time on the camera are correct before importing.
_If you are using a media card reader and are getting regular failures, try using a different reader or the camera itself (The same is true about importing directly from the camera. Try a reader instead.)
_Plug the camera directly into the computer with a solid connection, no loose cables. Don't use a hub.
_Reformat the camera's media card using the camera itself between uses.
_Alternate between multiple media cards during photo shoots (trips, etc) so that if one card goes bad (they are just little bits of plastic after all) you don't lose the entire shoot.
Hope that helps.