It seems to be that many people have the same problem, and DriveKey seems to be aware of that. They posted yesterday the following:
2009-03-09 >> Sporadic issues relating to original game discs
Over the last few days, we have been made aware of a problem affecting a small number of DriveKey chips in circulation. This problem only affects DriveKey chips with a W3 diode (next to the LEDs) when installed in a Wii with a silver drive cable. If you are able to play backup game discs but not original game discs, you may be affected. Please contact your point of sale for details on how to proceed and rest assured that the DriveKey team are 100% committed to solving this issue for everyone.
I have reported the issue, but I hasn't received any answer. I wonder if we are going to receive a replacement chip or if there is going to be an firmware update.