All the "bad news" is based on "correlations" and "associations."
There was quite a bit of investigation into "trans fat" a few decades
ago, and those studies did not find the major health risks being
claimed for "trans fat" today (see "Modern Nutrition in Health and
Disease" (by Shils and Young, 7th edition, or "Diet and Health" by the
National Research Council, for example).

If claims about "trans fat" are to be taken seriously, they need to be
rooted in science.  Would you avoid a food because a local religious
fantatic told you not to eat it because he  received a message from
God?  If you would, you can stop reading this post now.

So how can we define "trans fat" in a scientific way?  It's possible,
but only in an abstract way that is unrelated to health.  For example,
you need to have a fat source that has some unsaturated fatty acids in
it to begin with.  Then you force hydrogen into at least some of the
double bonds, creating an artificially saturated one.  So let's say you
have a highly unsaturated oil, safflower oil.  Now you do a very light
hydrogenation, and only convert a very small number of the double bonds
into saturated ones.  Is this now "trans fat?"  It's effects on your
body will be nearly identical to the safflower oil, with one exception:
you may have stripped all the natural antioxidant protection from that
oil (if it hadn't already been by the "refining" process, that is).
Now let's take another example: you take a fat source like coconut oil,
which is 92% saturated, and using hydrogenation, make it into 93%
saturated.  Is this now a "trans fat?"  It may be, but it is irrelevant
in terms of health, unless there is a toxic nickel content at this
point (nickel is used in the hydrogenation process).

You should be asking yourself, "why haven't experiments been done that
control for all these variables, and also contro for different
hydrogenation percentages?"  The answer is not easy, and probably
involves politics, sociology, economics, and psychology, but clearly,
the scientific method is being ignored here.  So let's say you
hydrogenate half of the double bonds in the fat source.  Would that be
like eating about half your fat calories from coconut oil and half from
safflower oil?  On the molecular level, is there any difference?  And
isn't that very similar to what most Americans are eating, in terms of
the kinds of fatty acids molecules, regardless of whether it is called
"trans fat" or not?

There is a key difference.  If you were to use fresh, unrefined
safflower oil, and eat plenty of antioxidant-rich foods with it, you
would doing something a lot less unhealthy than someone who eats
"processed" foods that have preservatives that basically stop working
when they are in your mouth as you are eating them.  That can be
measured scientifically, and often is (such tests are Rancimat and
ORAC, for example).

There is no argument on this point, for example:

"...reducing the proportion of unsaturated fatty acids which are at
risk of oxidation creates shortening that is less likely to turn
rancid. For example, a typical candy bar might have a shelf life of 30
days without use of hydrogenated oils, while the same product with
hydrogenated oils can last up to 18 months."

Source: http://www.answers.com/topic/trans-fatty-acid

It is also known that people who consume large amounts of coconut
product have very low rates of "chronic disease."  So it's not an issue
of double bonds by themselves, but double bonds AND a lack of
antioxidant protection, and that is the "killer combination" that
exists in most of the foods that are said (by the "experts") to contain
"trans fat."
Otherwise, how could it be "dangeorus?"  The same bonds are in all fat
sources.  We know coconut oil is fine, and it is highly saturated.  We
know that most "experts" are saying you need to eat quite a bit of
fatty acids that have double bonds.  "Trans fat" has an amount of
double bonds that is close to olive oil, which is being touted by the
"experts" as the "healthiest" oil.  As I've said in other posts, low
quality olive oil is very bad news, whereas the highest quality olive
oil is fine, but don't heat it or eat it if it has a rancid taste to
it.  Why?  Because if there are a lot of double bonds, they need to be
protected with antioxidants, and the high quality olive oil will have
quite a bit of squalene, which is a potent antioxidant, whereas the low
quality olive oil may have none by the time it reaches your mouth.

Many "experts" are talking about "trans fat" as if it is something from
a science fiction movie: "mutant fat from mars."  If you don't
understand or "believe" me, do your own experiment: get some mice and
feed half of them the cheapest vegetable oil you can find, along with a
standard "chow" that is low fat or no fat.  Feed the other half a
"trans fat" source such as "partially hydrogenated" palm kernel oil.
The fat should be around 25-30% of daily calories.  See which group
lives longer.

A new fat source, consisting of fully hydrogenated cottonseed oil
blended with "vegetable oil" that is highly unsaturated is now being
marketed as "trans fat free."  If my argument is correct, this fat
source should be no better than existing the existing major "trans fat"
sources, like margarines.  Again, if you were to give half of a group
of animals the new concotion, and the other half the old "trans fat"
laden margarines, there should be little if any difference, unless one
of them has a significanly higher amount of antioxidants.  So first the
Rancimat test could be run on these two fat sources.  If the Rancimat
tests were about the same, then you could do the experiment on the
animals.  Accoriding to the "experts" who are raging against "trans
fat," the animals on the new concoction should live longer and
healthier lives.  According to my argument, there should be little
difference, though the new concoction may actually be less healthy
(this has to do with a tangential issue, so I won't go into it here).

Again, you should be asking yourself, "why won't these 'experts' do
such simple and inexpensive experiments that would be conclusive?"

I don't know, but now, for a hundred dollars or so (the mice would cost
about $10 for a bunch of them), you could do it yourself and find out.