It may not have damaged other parts because they were powered from a different line from the power supply. The damage sequence would have been phoneline - modem - other PCI cards - power supply, and it never got any further than that.

This event is a good argument for having a surge protector with battery backup. I got a big APC unit on sale at Best Buy for under $50. The phone line runs through it, too, so no worries about the phoneline route any more. Cheap insurance.

You should have specified your motherboards make and model.
If we knew that we could look at a diagram and check how the board is layed out. Sometimes the part that controls the PCI’s is built into the south bridge of the motherboard, but if yours was then the whole south bridge should have crapped out. It is possible that your motherboard just uses a PCI controller chip that is connected on a bus that runs into your south bridge and that chip could have blown. I am no expert but I think I my answer is very close to the truth. A electronic engineer could probably tell you better.

EDIT:
Arg, I forgot the part about all the cards dying. That would most likely be due to a very high voltage that was applied to the cards. Any voltage in excess of the cards specs would destroy the card. If you put a working card into the system and it stopped working then your power supply might be the problem, that’s If you still have the same power supply in. In this case It probably was was not as a result of any external input to the PCI cards. Power distribution inside the PC is not my strong point.What you could do is take a multimeter and measure the Power supply’s output at the 20 pin connector.

I wish one of those Hardcore hardware pro’s would read this question.

EDIT:

Justin has told me by email that all the cards are suddenly working now. There was a GHOST in his new machine but it’s been exorcised. We’ll have to put this one in the X-files.