I wanted to update this thread because it's one of those evergreens that gets dredged up from time to time.
While 18 percent gray cards are still invaluable if you know how to use them, for most people it's now a non-issue because in-camera meters are much smarter than they were in the 1960s.
Here's an explanation for a typical metering system, this is Nikon's but they're all similar:
“3D Color Matrix Meter II takes into account the scene's contrast and brightness, the subject's distance (via a D- or G-type NIKKOR lens), the color of the subject within the scene and RGB color values in every section of the scene. 3D Color Matrix Metering II also uses special exposure-evaluation algorithms, optimized for digital imaging, that detect highlight areas. The meter then accesses a database of over 30,000 actual images to determine the best exposure for the scene. Once the camera receives the scene data, its powerful microcomputer and the database work together to provide the finest automatic exposure control available.”
For difficult lighting conditions, the best tool is still an incident meter, but a gray card works well too as long as you follow the directions which call for you to angle the card and use the exposure compensation recommendations that come with the Kodak-brand cards.
