I think the catch is that, being GPL licensed, any distribution of the software with modified source would be required to be open-sourced. With there commercial license you wouldn't have to release your code. (Though they list GPL, LGPLv2.1, and LGPLv3, you may or may not depending which license it actually is under and how you compile and run your program).

How exactly did they get around the GPL to offer a commercial license anyway? I would have presumed that people doing open-source work on Qt would have submitted their work under only the GPL, and re-licensing that code should be illegal since they didn't write it.

They have an agreement to allow all contributors' work to be relicensed as closed-cource.

Do you have to 'sign' or agree to something before they accept your code then?

Yes. In order to submit a patch though Gerrit, you must agree to https://codereview.qt-project.org/static/cla_individual.html Key is:

Subject to the terms and conditions of this Agreement, Licensor hereby grants, in exchange for good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, to Digia a sublicensable, irrevocable, perpetual, worldwide, non-exclusive, royalty-free and fully paid-up copyright and trade secret license to reproduce, adapt, translate, modify, and prepare derivative works of, publicly display, publicly perform, sublicense, make available and distribute Licensor Contribution(s) and any derivative works thereof under license terms of Digia’s choosing including any Open Source Software license.

I should note that one further reason for this is to conform with the KDE Free Qt Agreement (http://www.kde.org/community/whatiskde/kdefreeqtfoundation.p...).

Google does the same thing[0] whenever you contribute code to any of their open source projects.

[0] https://developers.google.com/open-source/cla/individual

One difference is that the Qt source code is also covered by the KDE Free Qt agreement (linked in a sibling comment), which basically acts as a poison pill to ensure that no future owner of Qt can pull an Oracle and close off development.

For this reason, it's my opinion that the Qt CLA is much, much, less onerous than other CLAs, like Google's or Canonical's, in the sense of how much control is actually given away.