If the moving air keeps on gaining speed as well, would it then create a blackhole like effect where it pulls surrounding objects into it?
I played the precursor, Narbacular Drop.
That’s some really special hay, btw. The tallest tower in Damascus is just ridiculously high and he somehow manages to fall on the little cart and it somehow stops him without damage. Leap of Faith, indeed.
not really a “black hole” effect, since that’s based on gravity, but given sufficient time it’s possible the air currents would build up to the point where at least small objects would get sucked in to the column of air.
An extension of this, since I’m not too familiar with the physics involved, would this downward air current allow for theoretically infinite velocity. If it was done in a room in a vacuum while wearing a spacesuit, would you simply infinitely accelerate at 9.8m/s, and would the velocity, or something, eventually kill you outright? (and if you could redirect the portal at some point, would this be, perhaps, a means to accelerate a space vehicle?)
BTW: it’s actually possible, given no other air currents in the room and the mechanics of the portal, to make sure you stay centered for an arbitrarily long time: namely, shoot the ceiling directly above you and the floor directly below you, and you’ll fall straight through, assuming the portal appears essentially instantly or at least uniformly around you.
A good question would be whether the system ever reaches equilibrium. The column between the portals is like an inexhaustible supply of potential energy, ready to add some free kinetic energy to anything that enters it and passes through the floor.
I suspect the air’s downward speed will rise to a high level, and the local temperature will also rise, as the moving air continuously collides with the relatively calm air coming from the rest of the room. When the air’s temperature rises to the point that it’s radiating away as much energy as it gets from falling, the air will stop getting faster and hotter.
Conceivably there could also be a “circulating donut” effect, where downwash from the column turns into wind blowing across the floor toward the walls, up the walls, then back toward the center. This is like what happens in a large pot of water when you put it on a small heat source, except upside-down.
I’m speculating here. (But intelligently, I hope.)
Lightspeed is one barrier of course, so it can’t get infinite. But I think there would be some much lower speed limit.
It’s never the speed that kills you. It’s the sudden deceleration at the end.
Of course they wouldn’t appreciate it! 
All the faller needs to do is place the orange portal on the floor next to the blue one, and then (after entering the blue one), place the blue one on the ceiling.
Person A is now ‘falling’ upwards and thus decelerating; at some point shortly before to after reaching zero velocity, they can grab the edge of the portal and escape, sans terminal slapping.
Better be quick on the trigger though, otherwise you’ll end up slamming into the ceiling at almost the same speed you were previously worried about running into the floor.
It seems to me that portals might make faster-than-light travel possible, because they effectively provide an infinite source of potential energy, as mentioned above. Infinite energy is just as illegal as faster-than-light travel, so why not allow both, within the Portal universe?
Or you could say that the infinite energy that seems to come from nowhere actually comes from within the gun itself, in which case it would have to eventually run out, and the portals would stop working. I guess.
I haven’t much to add, except that when I was playing I could just picture the potential physics/Portal Dope thread that might result, and now here it is.
“speedy thing goes in, speedy thing comes out” 
Is that how it’s purported to work? Or is it just conveniently unexplained?
They bring up a number of interesting issues - I think they could be used to create a reactionless drive (just have facing mini-portals in the engine and create thrust by accelerating a mass through them for a while - then flip the portals the other way and stop accelerating the mass, so it pays back its momentum, in the opposite direction, creating yet more thrust).
Also, in the OP’s scenario, the person looking through the floor will see through the ceiling, down on top of their own head, looking down through the floor etc - when this person jumps into the portal and starts falling, they will perceive themselves falling through a long tunnel, encircled by portals that go through a series of stacked rooms, but they will also see an infinite series of themself falling below and above. What’s possibly philosophically interesting about this is the question: which one of the infinite series of falling people are you?
All of them, I’d say; it’s not really that there are lots of you, just that light, travelling through the portals, happens to have a chance to bounce off of you in infinitely more places than usual. Much the same as standing between parallel mirrors.
Let’s not get into how the same question might apply to mirrors. I have enough difficulties every day without having an existential crisis while shaving in the morning. 
As TJdude825 says, you better have damn fast reflexes if you’re going to change the portals like that.
Assuming the floor and ceiling are the usual 8 feet or so apart, and that the victim is oriented vertically, is about 6 feet high, and that he’s falling at a speed of over 60 m/s — then he has less than 10 milliseconds’ worth of free time in each 50 msec cycle when he can change the portals without slicing himself in two.
Actually, if you play the game with commentary on, you’ll find out that they decided to never kill the player with the portals (a la Doom telefrag), or else they might get scared and not try all the fun things with them.
Assuming you place the first portal at the first opportunity, you have a leisurely quarter-second to reverse your aim to the ceiling and place the 2nd.
Ah. I haven’t actually had an opportunity to play the game yet.
So what does the game do if you’re standing halfway through the portals (both on walls, oriented vertically) and you move one of them to another wall? You’re not severed in half?
I think you just get shoved out of the one you’re in (ie, the one you entered from.) And there’s never falling damage, because you have some sort of apparatus on your legs that absorbs it all. You can still die, though, from falling into acid, or getting shot by a security robot, etc…