You could look at it like this. Think about the Video iPod (though completely different OSes, it gets my point across) Remember the Linux community for that? And if you ever screwed up anything, all you had to do was a HARD RESET IE a restore to fix anything gone wrong, you would have to do that when you updated through Apple Update too. So chances are pretty darn slim.
If there ever was an update, don't go with it right away! If you are afraid that itunes will search for updates, use it only when your internet connection is not active or disable itunes from connecting at all, or just use it on an offline computer.
Another method is if the update goes by and there are some good features on it, and either you wait for a crack to come out or you like the features better, restore your ipod and then update. The iPhone is a different story as it has the whole sim card issue and the fact that it doesn't work when you don't have a service plan for it.
So for the short answer, the chances are slim, I dare-say 1 out of 100,000.