An article appeared in the Wall St Journal today, here's a quote from it:

Pest Ground Planes

Every year, planes around the globe are held up by mice, rats, snakes, spiders and other unwanted stowaways. Insects and reptiles can terrify passengers, though they rarely pose a health threat. Airlines worry that despite back-up systems, some well-placed nibbling by a rodent could hobble a 100-ton, $200 million aircraft. Risks range from knocking out a reading light, to sparking a fire, or even crashing a jumbo jet by severing the networks for control, navigation or communication.

"If rats gnaw on a cable, who knows what they're going to do?" says Paul Hayes, director of flight safety at aviation consultants Airclaims PLC, in the United Kingdom.

Just another good reason to De-Rat the plane before take off.