NTodd quotes Slate noting that "Last year, Limbaugh, who used a tailbone defect to get out of the Vietnam draft, accused a Democratic candidate of having served in Iraq 'to pad the resume'." But commenter Interrobang corrects the record.
Rush didn't get deferred from Vietnam because of a "tailbone defect," he had a pilonidal cyst, which is a hair-filled sinus on the skin over the coccyx. The problem is entirely within the skin, and has nothing whatsoever to do with the tailbone itself.
So I looked this thing up (can you tell I'm procrastinating? I have a paper to write.) and it turns out that a pilonidal cyst is a little hairy thing located about here:
Ergo, a tail.
It actually explains a few things, including his being in league with Satan.