To be honest man, its just something you gotta deal with. Ive learned to always watch my back. Pretty much the best thing you can do, really. Especially a couple seconds after you spawn. Whenever you spawn and start running to where ever, after you turn the first corner, turn back and check around the corner before you move on. A lot of the time you'll get a guy who just spawned where you did, and voila, free kill! Or you can do what i did and pretty good at somewhat predicting where the enemy is gonna spawn based on the lay-out of your teammates on the mini-map. The only 2 tips i got for protecting your a*ss from the dude spawning behind you.
I haven't noticed if it has been changed lately. I think they change things every now and then, but it mostly only changes whether you spawn behind the guy that killed you or in front of a guy.
And no, the problem isn't just the map size. Small maps in MW1 and MW2 didn't have the issues that larger maps in MW3 have. The problems are a combination of factors.
There aren't enough spawn locations. People can die fast, and can die in groups. As well, there are more killstreaks and more equipment to interfere with or capitalize on spawn locations. You know the system has some serious issues when FFA spawns two or more characters in the same location.
The map designs tend to limit and funnel movement. If you have only two directions you can go, and an enemy is near you, then you have a 50/50 chance of running into them. What would theoretically be a "safe" distance from another player might not be so safe when you consider the limited directions of movement allowed there. And half of those times, someone is going to be running into someone else's back. Similar goes for when views are limited, where while you can see less of the map, it also means you can cover the part you can see better. Maps in previous games used their space better, in ways that allowed more players to be within a limited area without immediately or near immediately being in each other's sight. MW3 greatly simplified interiors, removed a lot of verticality (which reduces movement routes and makes it easier for one person to cover his limited area), changed how obstructions were placed, and a variety of other things.
The spawn locations are poorly placed on the maps. This plays into map design, because some maps don't really have many, if any, good places to actually put spawns. Put them in a travel path, and someone is going to get shot in the back. Put them next to travel paths, and the spawner is going to either wait and shoot someone in the back, or step out and get shot. There is an added complication that a "safe" spawn location can look like an appealing sniper/ambush location as well, which leads to people spawning behind snipers. (Theoretically, the game wouldn't spawn someone behind a sniper, but when the system is failing everywhere else...)
The game doesn't adequately realize whether a spawn location is "safe". Take the loading bay area in Dome, where a guy could stand outside next to the truck and spawn kill. Or the long underground section in I think Underground, where you could stand and spawn kill guys spawning at the other end.
Some game modes make spawning more problematic, and can result in choosing between two undesired results (such as choosing between easier spawn trapping or introducing more chaos). It doesn't help that Domination also has issues with its flag placements.
And while some people say "Spawn guys away from the person that killed them", that can itself lead to issues. At its worst case, basing spawns on player positions can make it extremely easy to predict where someone will be spawned. You can see this in 1v1 matches, where if you pay enough attention, you can start to judge and even manipulate where the game will spawn your opponent. ("Spawn guys away from the person that killed them" isn't always desireable anyway, just as "Spawn next to an ally" can lead to fast double kills or expose a guy who was being stealthy, or "Spawn next to the action" can lead to fast unwanted deaths. For example, in a FFA match, spawning away from your killer can spawn you away from most action, which acts as an extra time penalty on people who die. Facing a sniper, spawning away from your killer can make you vulnerable to the sniper again, made worse if said sniper has an adequate view of the likely far spawns.)
An excellent and informative reply Soracia, I never knew so many variables had to be considered when designing spawns.
I have enough experience in MW3 to know the spawns and know the probability of enemy spawns based on killfeed and friendly positions. But recently on KC,TDM and Dom its felt like release day spawns. There are people spawning directly behind me, like 5 foot away. One moment they are looking at me killing them then *blink* they have spawned perpendicular to me watching me then *BAM* they get insta-payback. The spawn is so fast an close that their previous body is practically just hitting the deck before they re-appear 5ft away. These incidents have become noticably more frequent. Oh well, one day it may get better. :)