In an article about the SQL Slammer virus, Nathan Hanks, the CIO of Continental Airlines, said this:
If those ports weren’t open, slammers would be a non-event. We have MSDE on every single gate PC in our hubs. We immediately failed at putting people on planes.
Very interesting. MSDE is the Microsoft SQL Data Engine — basically a black-box version of SQL Server that runs as an executable, enabling it to be distributed with apps. It has no user interface and has to be completely manipulated via SQL.
Given the quote above and a little guesswork, it would seem that Continental has compiled workstation apps installed on all their gate terminals and are running transactional replication back to the server. The replication probably goes both ways — changes made at one gate terminal are replicated to the server which then replicates them out to the other gate terminals.
This is a good way to protect against network outages — each gate terminal is autonomous and can survive an occasional lapse in connectivity to the server.