First of all, the top as it relates to the movie:
Technically, when the screen goes dark, the top is still spinning.
However, if you’re like me the first time I saw the film, I “knew” the top was going to fall. He just cut at the last second, as a final “gotcha!”
Then, on the way to the car, my wife remarked how she thought it was lame that the top was “obviously” going to spin forever and it was all a dream.
We can’t use the image on film as “proof” that it’s reality, because the last image we see on screen is the top spinning.
Wobble or no, if there’s a finish line, the “Ooooh, he’s still in a dream,” folks made it.
Or did they?
They only way to be really sure that the top spins “forever” is if the film lasted forever. It does last a really long time, but who’s to say it doesn’t fall the instant we cut?
The top doesn’t hold an answer, it asks the question.
Secondly, regarding the mechanics of the thing:
Because Christopher Nolan avoids CGI whenever possible and is familiar enough with illusions, I’m pretty convinced that the perpetually spinning top we see is a variation of Top Secret.