What do you mean by open source? Do you want the banks website to be open source, or all of the back office code that talks to ancient mainframes and proprietary 3rd party applications?
What will draw customers (other then fellow developers) to a bank with open source software?
Good Question. Lets take the scenario where we are partnering with another bank. So then we have two main applications or systems - 1) Integration with their api (I'm assuming this would be a fairly big thing considering the complexities) and 2) Our API/App i.e what people would actually use. I would see the second one there being open source, but not the first one, for obvious security reasons. Furthermore, apps and other smaller services would be open source. I think if you can portray an image that you are transparant and in a non technical way, have it as part of the brand as such, it might interest people. Get me?
What will draw customers (other then fellow developers) to a bank with open source software?