Is your cabinet really that unstable? I have a 100# 60" plasma sitting on a stand and it would take a lot to get it to fall over.
If you really want to tether it get creative. Unless you're going to climb on it you don't need much as you just have to keep it from going past the half way point. A light string or cable tied to the center of the set should retain your ability to turn it.
My cabinet is fine. But the tv HAS to be tethered, according to the home study requirements. (I'm going thru adoption).