Looking at the SRD the spell "Mage Armor" reads as such:
An invisible but tangible field of force surrounds the subject of a mage armor spell, providing a +4 armor bonus to AC.
Unlike mundane armor, mage armor entails no armor check penalty, arcane spell failure chance, or speed reduction. Since mage armor is made of force, incorporeal creatures can’t bypass it the way they do normal armor.
I would like to add on cavaet to the description of the spell:
All natural effects from a spell or spell-like ability still affect the subject of the mage armor spell.
What does this mean? It means if magical ice, alchemist's fire and other such things are used against the subject of mage armor the subject avoids the damage of the spell but may be damaged by the natural effects of the spell or spell-like ability. This means, the heat of the alchemist's fire may cook the subject of the mage armor spell the way toast is cooked by a toaster oven. Likewise the intense cold from magical ice may cause frost-bite.
Just a thought.