If I distribute the Coin source code, under what license is it for my customers?

Under the GPL and PEL. The license is connected to copyrightable material (in our case, the Coin source code), not the modus of distribution. So regardless of how you get the source - from SIM, from somebody else's website, from a Debian CD, from Santa Claus, it will always be under both the GPL and the PEL.

That means that your customers can use the Coin source for Free Software development as covered by the GPL. For any proprietary development, they must acquire a PEL.