In summary there are 2 schools of thought:
School 1: Microfractures don't exist in the type of (relatively) soft aluminium alloys used in climbing carabiners. Supporters of this stance point out that the 'myth' started out when an El Cap climber somewhere in the 70's dropped a handled ascender (Petzl jumar) onto the rocks below a pitch, then while using it, the twisting motion of a locker in the lower clip in hole with a small/hairline crack running through it resulted in the footloop connection breaking off. Please note that the technique used to manufacture this jumar was not forging (hot or cold, as is used in most modern aluminium climbing gear manufacture) but rather
casting - a process that results in harder but more brittle aluminium. A study or two [citation needed] has found that these cast parts are indeed susceptible to microfractures after an impact force (e.g. dropiing from height onto a hard surface), but nothing on my rack is cast.
As I remember, this climber didn't die (well of that anyway). This however gave rise to the spread of the whole microfracture thing. Most people who'll tell you that your carabiner is fked beacuse you dropped it have never seen a microfracture, in carabiner alloy or any other metals, or even know this little piece of history. Supporters of the first stance say that microfractures are a self-perpetuating fallacy: everyone telling each other how to avoid the dangers of this thing that just about no-one has ever seen. If you google around a bit, and sift through all the bullcrap, you'll find the report a couple of guys wrote about microfractures: they picked up over a hundred carabiners from the base of El Cap, dropped from the numerous routes above. None of them broke under their rated strength.
If you have a cast carabiner, please post a photo and don't use it if you dropped it from height. If you started climbing in this century, chances are you don't have any.
School 2. Crazy people.
There, balanced analysis for you.
Disclaimer: If I die because a microfractured carabiner breaks, don't believe the first part

If that doesn't put your mind at ease, I recommend the recipe I posted earlier.