In Diablo 1 and Diablo 2 there where some very powerful randomly generated items that could not be repaird as their repair costs were way too high. Is this also true for Diablo 3?
No. Ethereal Items and the like are not present in Diablo III.
All items can be repaired as their durability decreases. There are however, some items that do not suffer durability damage, and this never need to be repaired.