Today I discovered an application that is using Qt4. It was a complete surprize, as this is a software that you can download from a photo-shop's web page to create so called "photo-books". Unfortunately it is a Windows only application, but I was happy when I saw that QtCore4.dll & others were installed (inside wine). 
The application is in Romanian, but as I saw it was developed in Germany. It is called CEWE Fotocartea Mea (CEWE My photobook) and can be downloaded from here.
A commercial Qt application
I note you've only linked to
Submitted by cmorrell on Thu, 07/19/2007 - 11:38.
I note you've only linked to the download for the application and not the service, so can I ask you to confirm if this is the same service/software please?
Same service
Submitted by amantia on Wed, 07/25/2007 - 20:26.
Yes, I linked to the application, because the service page was in Romanian. But yes, they seem to be the same thing. Just for the record: http://asp08.photoprintit.de/microsite/fotobook_details.php
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A other commercial Qt 4 application
I found a other commercial Qt 4 application
Autopano Pro
http://www.autopano.net/
A windows and mac version.
A linux version is promised