There are some materials, for which it decreases. It depen ds of a coefficient, called "alpha" (okay, abreviated alpha, i don't remember the constant name, it's some sort of t emperature coeff I think).
This website states it more eloquantly than I can: http://matse1.mse.uiuc.edu/~tw/sc/b.html
"Metals:In a metal, the valence electrons are thought of as being shared by all the positive ions. Therefore, the electrons are free to move throughout the crystalline lattice. The electrons move randomly throughout the crystal, until an electric field is applied to the material. Then the electric field forces the electrons to move in a direction opposite to the field. Actually, the electrons still move somewhat randomly, but with a superimposed "drift". This produces current. As the temperature increases, the positive ions in the crystal vibrate more, and more collisions occur between the valence electrons and the vibrating ions. These collisions hinder the "drift" motion of the valence electrons, thus reducing the current. In summary, for a metal, an increase in temperature causes an increase in resistance."
hmm, this might be repeating what neuronix quoted..
But here's what i'd assume the reason:
Resistance is caused by electrons hitting the particles (atoms). At a higher temperature, the atoms are moving about more, so they will collide with the electrons more, thus creating a higher resistance.