lubrication
According to this answer WD-40 is not a true lubricant. I don't get it – even Wikipedia says
The long-term active ingredient is a non-volatile, viscous oil which remains on the surface, providing lubrication and protection from moisture.
which sounds quite solid.
What exactly does WD-40 lack that prevents it from being a true lubricant?
Properly lubricating the hinges should stop the squeaks. In short, you remove the hinges from the door, lubricate them, and put them back.
You'll want to use white lithium grease. It works very well for metal on metal contact.
I'm assuming simple door hinges here, like these
You'll want to do each door one at time.
Long Process
Short Process
Do one door first, and give it few days to make sure the squeak is gone.
After reading @woodchips answer and throwing in my $0.02 offhand, I did a bit of research. I found this article on the use (or non-use) of graphite on aircraft. The article mentions that metals such as aluminum, magnesium, zinc, and cadmium may experience galvanic corrosion when exposed to graphite lubricant. Since galvanic corrosion requires an electrolyte, you might be okay if your metal components are inside, and not in a wet area.
The article also mentions that graphite can damage stainless steel at high temperatures. (Probably not going to be an issue in a home though. If it is, you may have other problems like a house fire, and probably won't care much about your hinges at the moment.)
Best Answer
WD-40 is composed of many different chemicals, and only about 10-20% of these are lubricants. Even then this small amount is only a very light mineral lubricant. Most of the other components are intended to help penetration, but are volatile and are intended to evaporate.
WD-40 does provide some longer term lubrication, but the problem is that the volatile components will dissolve and displace whatever lubricant already existed (e.g. grease), and then the majority of the WD-40 will evaporate. So really it should not be used on anything that requires propper lubrication (or at least, a proper lubricant should be added again some time after using WD)