Revving your engine when you first start it up and it's not warm yet will cause premature wear on internal engine parts. This is because after the car has sat overnight or for a substantial amount of time, the oil has had time to drain out of the top of the engine and back down into the oil pan. It takes a minute or two of the engine running to get the oil pumped back up to the top of the engine to start lubricating the moving parts properly. Also the metal parts haven't had enough time to expand properly from temperature change to their operating tolerances.
Also, if you are holding a steady idle (1000-1600 RPM's) it will not hurt your engine in fact, some companies program their ECU's upon startup, to hold around a 1300 RPM level to let the engine properly warm up. Anything in extreme excess of this threshold is destructive to your engine.
As a rule of thumb, until the engine starts to sound/feel normal, I do not drive it.
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-JWT
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