<p>We maintain a system that has over a million lines of COBOL code. Does someone have suggestions about how to migrate to a GUI (probably Windows based) without losing all the business logic we have written in COBOL? And yes, some of the business logic is buried inside the current user interface.</p>

Writing a
screen scraper
is probably your best bet. Some of the major ERP systems have done this for years during a transition from server based apps to 3-tier applications. One i have worked with had loads of interesting features such as drop down lists for regularly used fields, date pop ups and even client based macro languages based on the scraping input.
These weren't great but worked well for the clients and made sure the applications still worked in a reliable fashion.
There is a lot of different ways to put this together, but if you put some thought into it you could probably use java or .net to create a desktop based application and with a little extra effort make a web based implementation.