Loggerhead,
We did an extensive study of this at my airline and found out that the fuel savings were more than offset by wear and erosion in the remaining engine. Any time you take a jet engine above idle on the ground you start sucking up sand and debris. Even if you don't get the classic fod damage, all the sand that goes through the engine erodes the compressor seals and whatnot making the engine "Leaky" so that it burns much more fuel in flight.
The only time one engine is shut down now is if we are sitting in the penalty box waiting for a gate that is more than 20 minutes away.
Cheers
Wino