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In a typical modern, heavily computer controlled, somewhat complex passenger car, is there any difference between pressing the gas pedal vs. opening the throttle valve under the hood by hand?
E.g. important sensors on the pedal or the cable, other cables that aren't actuated when opening the valve directly, etc.?
If the engine is controlled via cable (drive by cable) directly from the gas pedal, then there is no difference if you were to open the throttle by hand and open the throttle plates. In fuel injected cars of this type, there is usually a throttle position sensor which tells the computer how far the plates are open and adjusts the other engine components accordingly.
The caveat I'll give you though, is that most vehicles today are drive by wire, meaning, there is a rheostat attached to the gas pedal which interprets your intent as to how fast the engine should be going. This signal is fed into the ECU, which in turn opens the throttle plates and adjusts the engine accordingly. This doesn't mean the throttle plates operate exactly as you push or release the pedal. Since it's controlled by the ECU, the ECU decides when and how far the throttle should open or close. The throttle usually does follow pretty much what you are telling the gas pedal, but that doesn't mean it does it exactly. The other thing about it is, with drive by wire systems, there's nothing to turn or grab hold of with the throttle plates. There's a motor which turns the plates and is self contained. You cannot poke your head under the hood, grab hold of the throttle, and give it a whirl. It just isn't possible.
As fair as I know, it is the same since the wire turns the throttle valve. The commanding wire doesn't do anything else, nor it has any other sensors/wires attached. It is a simple mechanism to act over the throttle. The ECU (computer), may adjust the fuel/air mix by acting over other electronic valves and mainly depending on the exhaust gases "quality", among other readings it uses.
The short answer is "It depends". Paulster explained the most common caveats of drive by wire throttles, but my car is quite a bit different than that...
In the early 2000's, BMW introduced a variable valve lift system they called Valvetronic. That system almost eliminates the throttle plate altogether, favoring an open throttle and controlling power output by varying cam timing and lift. The ECU translates the throttle pedal position into adjustments in all those controls to make it behave the way the driver would expect a traditional throttle to work.
Given the complex interactions occurring in a system developed in the early 2000's, when drive by wire was still relatively new, I can only imagine the engine management in modern vehicles is even more complicated. If you manipulate the throttle plate manually on a modern vehicle, expect unpredictable results.
