First, I take issue with the premise of your question a little bit--just because one could hypothetically shift to neutral does not mean "there is no such thing as uncontrolled acceleration." How often do you think the typical driver of an automatic transmission car selects neutral? I would think that a driver could go years without having the need to shift to neutral. It is reasonable to expect a state of panic to take hold of a driver who is behind the wheel of a car that suddenly appears to accelerate on its own. In such a state of panic, it may be a stretch to expect this driver to think of shifting to neutral to halt the car's acceleration.
There are potentially a vast number of ways to bring a car that is accelerating on its own under control (use the brakes--they're more powerful than the engine, switch off the ignition, change to neutral, depress the clutch), the problem is thinking logically and reacting quickly enough to prevent a problem when you're suddenly in a panic.
That aside, to address your question. If a manual transmission car is accelerating HARD, as might happen if the accelerator was stuck full-on, it may require quite a bit of force to move the gear selector to neutral (unless the clutch was depressed first). When the transmission is loaded, it kind of holds itself in gear. The same may be true for some torque converter automatic transmissions, though I wouldn't know.
There are a variety of transmission types in use now:
- Conventional Manual
- Torque converter automatic ('contentional' automatic)
- Computer-controlled manual transmissions with no clutch pedal (e.g VW/AUDI's DSG, BMW's SMG)
- Continuously variable transmissions (CVTs)
While it might be physically, theoretically possible to shift all of these to neutral at any speed (i don't know that this is the case), modern transmissions have fancy electronic controls, and use a wide variety of levers, knobs, buttons and switches for mode selection. It may be possible that the control software does not allow the driver to select neutral at speed.
Likelier still is that the driver simply wouldn't be able to figure out how to do it quickly enough in an uncontrolled acceleration scenario.