I know I read the KDE version somewhere ... It's section 10 of the KDE
FAQ
=============================================================================
0. Miscelleanous questions

10.1 Why does KDE use Qt? 

Qt is a very sophisticated toolkit that provides everything that is
needed to
 build a modern user interface. Qt is written in C++, thus allowing
object
 oriented development affording the necesary efficiency and reuse of
code
 necessary for a project of the enormous size and scope of KDE. In our
opinion
 there is no better toolkit available for Unix and that it would have
been a
 grave mistake to try to build KDE on anything but the best. 

 Furthermore, all KDE developers agree that it would have not been
possible to
 build KDE in equally short amount of time without the use of Qt. 

10.2 Why does KDE not use gtk, xforms, xlib, whatever? 

There are a number of toolkits available. To provide a consistent user
interface and to keep used resources such as memory to a minimum, KDE
can 
use only

one of them. Qt was selected for the reasons mentioned above. 

10.3 But Qt isn't free, is it? 

That depends on your definiton of free. According to our definition it
is
 free, since: 

    The Qt libraries and source code are available for free and are
readily
 available on many ftp site. 
 You can run application written using Qt for free. No run time charges
 apply. 
   You can develop application with Qt for free as long as you make the
 source code of your applications written in Qt readily available. Note
-- you
    can even sell your application, however you must make its source
code
 readily available if you wish to avoid buying a license for Qt. 
    You can distribute Qt as well as application using Qt in any form --
be it
 on ftp sites or CDs. In particular all Linux distribution may press a
copy of
 Qt
    and its source on their CD's free of charge. 

As KDE was, and is, intented to be a noncommercial desktop environment
for the
X Window system, Qt fits our purposes. 

Qt is constantly developed by Troll Tech's dedicated, professional
staff.
 Troll Tech devotes all of its resources to improve Qt on a daily basis.
Due
 to this fact
Qt continues to develop at a rate unmatched by any L/GPL'ed toolkit,
providing
 us with the best toolkit available under Unix and providing possible
commercial developers for KDE with the quality and support they need. 

10.4 Why not code a L/GPL'ed Qt clone? 

Given the threat that Windows NT poses to Unix we feel a strong sense of
 urgency and believe it to be important to develop a great desktop for
UNIX
 first,
before we might even want to think about rebuilding the _tools_ we are
using
 to build this desktop. Given the fact that our resources are limited
KDE
 itself can
not engage in a Qt cloning project at this point. However, if anyone
feels
 that they would prefer a L/GPL'ed version of Qt we would like to
encourage
 you to
write such a L/GPL'ed clone. Keep in mind that Qt is only about 80.000
lines
 of code while KDE is already more than 500.000 lines and the number is
rising
daily so that a precise count is very difficult. This means writing a Qt
clone
 is definitely feasable and it is only a matter of people who feel
strongly
 enough about a L/GPL'ed version of Qt getting together and starting
such a
subproject.