My SIM is not recognized after installing Cyanogenmod; what should I do?
OK I feel incredibly stupid now.

I had entered the SIM card upside-down (the "cut corner" was looking "down" instead of "up"). Once I rotated the card 180 degrees on the z axis everything seems to be dandy.

In my defense I must say that the the I9000 SIM port doesn't enforce the "correct" SIM position in any way; it has a small engraving that I just didn't see when removing the card the first time, and that's all. But the card fits both ways.

I will close this question as soon as I'm able to.