I have just read the Theory of Karma in Buddhism thread by AEN.
Rather lengthy I would say.
http://www.sgforums.com/forums/1728/topics/387044
Here's an extract from 'Only We can Help Ourselves' by
Dhammavuddho Thero.
Kamma-vipaka is sometimes translated as cause and effect ('yin
kuo' in Chinese books) but that is not a good translation.
This is because there are two types of cause and effect -
worldly cause and effect, and
kammic cause and effect.
The difference can be seen, for example, when you drive a car
and suddenly a small boy runs across the road, and you hit the boy
and kill him.
You did not have have the intention to kill the small boy, so
you did not create any kamma. However you did knock down the
boy and the boy's family might pull you out of the car when you
stop, and beat you up. In addition to that, they might also
sue you in court.
So there is a result of knocking down the boy, namely, you get a
beating and then be sued in court. However this is worldly
cause and effect. It is not concerned with Buddhist kamma
vipaka because there was no intention on your part to kill the
boy.
So we must clearly differentiate between worldly cause and
effect and kamma-vipaka. They are different because one
involves action with intention and the other does not.
...The working of kamma-vipaka is very
complex. The Buddha said in one discourse (Anguttara
Nikaya 4.77 - Acintita Sutta / 增一阿含經，分别功德论卷第一)
that there are four things that you should not think about too
much. If you think too much about these four things, you
could become deranged.
The first is the power of the Buddha;
The second is the depth and power of jhana (meditative
absorption);
The third is kamma-vipaka;
And the fourth is speculation about the world.
...The working of kamma-vipaka depends not only on our present
lifetime but also on many, many lifetimes of unsettled
kamma accounts.
On top of that, it also depends on the state of our mind at that
moment. Moreover, the state of mind of the other being you do
the kamma to also affects your vipaka.
For example, when you make an offering to a murderer, or a
criminal, or to an evil person, your merit is very small, but if
you make an offering to a holy man, then your merit or blessings is
great. Conversely, if you kill a holy man, then your kammic
offence is very great. If you kill a mosquito, the kamma is
not as bad, but still you are not encouraged to kill it.
...Sometimes we see certain people whom we consider as evil,
rough, stingy and offensive, but they are having a good, luxurious
life. That could be due to their good kamma from past life
supporting them. However they are now creating a lot of evil
kamma which has not ripened yet. This could be due to
the fact that people can change.
For example, when a man is poor he is humble, industrious,
faithful to his wife, frugal, etc. When he becomes rich after
several years, he may become arrogant, vain, womanize, drink,
gamble, etc. - a completely different personality.
Similarly, a good person in a previous life, when reborn under
favourable conditions, might become corrupted by his good
fortune. Because of this we may find it hard to see fairness
in this world.
Sometimes we see that good people are not the ones who are
happy, but many evil people are happy.
The working of kamma-vipaka is very difficult
to see unless we have the psychic power to look into the
past.