Pie Bald + Normal.

[towelie4365]

Like sho220 said, they would be het for pied.

A bit more about the genetics:

Piebald is recessive, meaning you need two of the genes to physically display the trait.

So say AA was piebald, and BB was normal, if you bred them together, all the babies would only have AB for their genes, which is considered "het", or heterozygous, for that gene.
If you bred a piebald to a het for piebald, it would be AA cross AB, which could give you the four possible outcomes of AA, AA, AB, AB, meaning you have a 50% chance at a pied, 50% chance at a het for pied.
If you bred two het piebalds together, thats AB cross AB, giving you AA, AB, AB, BB. So you have a 25% chance at making a pied, 50% chance at making a het for pied, 25% chance at making a normal. Since you can't tell a het from a normal, then you just say its a 75% chance at making a 66% het for pied. That means theres a 66% chance that normal looking one is het for pied.
The easy way to explain how to get the possible outcomes is in whats called a Mendel's square:

In this case it talks about albino, but its also recessive like piebald, and uses A for non albino, a for albino. The p just stands for probability, showing the 50% and 25% chance I talked about.

Also, piebald to piebald gives all piebald.