So I glitched out of hidden, and went all the way to the Chernobyl station. There, I fell below the map, into a black area. Usually you die when this happens, but for some reason I didn't. So, I decided to go to the objective (Now 800m away) but I hit an invisible wall somewhere along the way. So, I walked to the edge of the black area, where the background met the horizon. To my suprise, I began falling. Indefinately. So, rather than quit, I decided to use this oppertunity to figure out how fast you fall in MW2.
Looking up, I saw that the objective counter was rapidly increasing it's number, albeit at a CONSTANT rate (Let's forget about -9.8 m/s/s, since this is a game) So, I started a stopwatch to see how much distance I would gain in 60 seconds. That number was roughly 240,000m. (I'll have you know, the counter was around 800,000m when I started, so Terminal Velocity has most definatley been achieved) So, Divide by 60, and you'll find that MAXIMUM falling velocity for MW2 is 4,000 m/s. Hooray!