IAS on weapons increases that weapons overall DPS, resulting in a pure DPS boost?
Any other IAS increases sheet DPS, but this is only estimated due to more APS, and not an actual increase in damage.
Thus having more IAS on non-weapon gear is only effective as a DPS increase if you are actually spamming that attack constantly. IE holding your mouse-click down while casting magic missile will result in maximized damage due to attacking at your max APS, while casting frozen orb and then waiting for arcane power to regen does not result in any real significant DPS boost, since the higher APS is negated due to the fact that you must wait for your arcane power regen. (or for another example, kiting a monster with a ranged spell negates the APS boost since you are not attacking at your maximum APS)
IAS increases ticks per second on any channeling spell, and thus increases overall damage on channeling spells (while increasing resource cost)