Just thought I would add some info I've found on identical twins:

From Yahoo:

Identical twins, formed when one fertilized egg splits, are the only people in the world with identical DNA. Fraternal twins, on the other hand, are formed when two different eggs are fertilized. Genetically speaking, fraternal twins are no closer than normal siblings, sharing only about 50% of their genes.

Although identical twins have the same genotype, or DNA, they have different phenotypes, meaning that the same DNA is expressed in different ways.

Traits determined by phenotype, such as fingerprints and physical appearance, are the result of "the interaction of the individual's genes and the developmental environment in the uterus." Thus, a DNA test can't determine the difference between identical twins, while a simple fingerprint can.

From Wikipedia:

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The two embryos develop into foetuses sharing the same womb. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing foetus. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. In rare cases, blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome.

Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. They look alike, except that they are sometimes mirror images, but examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.

It is thought that identical twins do not run in families, but occur more or less randomly.

Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). They develop their own individual personalities to enable themselves to be identified as individual persons. Many identical twins spend most of their time together (especially as children), so people can assume that they will behave alike just as they look alike; however, this is not necessarily the case. Twins establish their own individual likes and dislikes. This is not to say they are totally different, but there are usually obvious signs of differences when the identical twins are observed separately or together.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigentic modification. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over 3 times the epigentic difference that the 3-year-old twins had. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.