A friend's DSL modem died during a brief power outage. The voltage surge and/or voltage sag can easily damage electronic components.

A more ominous possibility exists that the mac's ethernet circuitry has been damaged by the power fluctuation. This happened to my iMac. The external DSL modem was fine but the ethernet had died. To get around this, I purchased an Airport basestation and card to connect wirelessly to my DSL, thereby bypassing the ethernet (and believe it or not, saving $$$ over what the ethernet repair would have cost me).

Methinks an Uninterruptible Power Supply (UPS) with a phone line surge suppressor would constitute a wise investment.