Just curious if this has ever happened to anyone. I know people who like to put objects behind the shifter, heavy enough to slide the shifter to neutral. So it got me thinking, if this happens, can they just shift back into drive going, say, >10mph without damage? or must they slow down to a crawl. My gut tells me they have to slow down, so the gear can catch.
It is fine to just put it back into drive. You probably don't want to do it repeatedly because you could maybe wear out some clutch packs in your auto trans but for the most part it is much safer to just throw it into drive and get back into the flow of traffic than coasting and pulling over to stop then back to drive.
When you put the car into neutral at speed the output shaft is decoupled from the input shaft. This means that they can start rotating at an unmatched speed. To go back to drive this speed needs to be resynchronized.
There are three components of the auto trans that should protect it from damage.
1) The torque converter - The torque converter spins at engine speed and the input shaft can rotate independent of that speed (equal or even much slower). Since the input shaft can rotate fairly independent of the engine it should not provide a huge amount of force if you try to change the speed of the input shaft to match the output shaft (which happens when you go back to drive).
2) Clutch packs - Most auto transmissions contain clutch packs which engage /disengage to enable or disable certain rotations and therefore certain gears. These work similar to any other clutch in that they will allow some slip as they engage cushioning or smoothening the engagement of the gear. This is what will clamp down to try to synchronize the input and output shaft. The two halves of the clutch pack will be rotating at different speeds when in neutral then when you go to drive the clutch will be engaged and they will synchronize. These will absorb most of the force and the torque converter will also absorb some and prevent the load of the engine from adding too much more force.
3) Accumulators - Auto transmissions contain large hydraulic cylinders which are designed to cause the hydraulic fluid to build up pressure gradually or smoothly instead of slamming immediate pressure to the components. These should allow the clutch pack to engage smoothly instead of rapidly causing undue stress on other components.
I'm not aware of any automatic transmission cars that even have a flywheel.
One of the noise isolation procedures that is used by mechanics involves getting up to highway speed, knocking the car into neutral, then knocking it back into drive (while comparing the sounds between the 2). I can't guarantee that it's safe on any particular car, but as a general concept it doesn't seem to cause any harm.