I thought there was a $1 per share minimum....they had talked about repealing or amending that rule recently...not sure if they actually did it.
If the rule is still in effect, apparently the price has to be below $1 for an extended period, not just one trading day.
http://rturner229.blogspot.com/2008/08/new-york-stock-exchange-delists.html
As I understand it the only reason the stock is this HIGH is because index funds have to own shares of every listed stock. That provides a little demand. Once the stock is delisted, or GM goes bankrupt (whichever comes first), the stock will be worthless.