Copyright when working with Open Source?

I have a question about copyrights when it pertains to making modifications to an open source script. I understand the need for the developer (me) to retain the rights to the scripts created, but what about these two examples.

1) I take an existing payment module for osCommerce and create a derivative to work with a different payment processor.

2) I create a custom theme for a Wordpress Blog.

Both osCommerace and Wordpress are released under the GPL. From what I understand, any derivatives (1) must be released under the GPL and any programs that interact with a GPL program (2) must also be released under the GPL.So, I would remain the copyright holder of the modified payment module and theme, but they would be required to have the GPL license instead of what would be covered in the contract? Does that create any possible legal issues for me?

Are there any other implications in working with open source software vs all proprietary?