The knowledgebase indicates that the cause of missing rubber or exposed wires must be a case of abuse, which is so ridiculous for most of the people posting about this issue. For one thing, the computers aren't that old and how on earth would they "abuse" the power cord. Wouldn't you expect these cords to at least "hold up" during normal use? I can't imagine Apple not replacing them instantly to avoid a more serious problem.
2.5 G5, 60GB iPod Mac OS X (10.4.8)