Nowhere in the specs of the Sony Ericsson C720 does it say that it's completely waterproof—just dust and splash proof. And if there's one thing that a toilet does well (at least when we use it), it's splash. This could be one of the more disgusting tests imaginable for a phone. Seriously, are you going to use it now, because the CDC thinks otherwise.
AU: I wonder if flushing it would help me open the back cover?