I've seen already three different developers asking on IRC: "Why do you use Subversion in the Trolltech labs to publish your source code, instead of $FAVOURITE_REV_CTRL_SYSTEM?". To make everyone's life easier I thought I'd just answer the question here :)
The two main arguments are simple:
It's portable. It runs well on Linux, OS X, Windows and more. That's a must for us.
It's easy to use. It's really simple to use for our purpose and the documentation is very good, too.
Now it may not be as powerful as $YOUR_FAVOURITE, but that's not really important for us, because we use it just to share source code. The actual development happens in our Perforce repository, and we use a little script to automatically convert the individual Perforce changesets to Subversion. That's convenient for us.
But Subversion can also be very convenient for you! If you're an open source software developer using Subversion then it's really easy for you to use a project we have in the Trolltech labs in your program through svn:externals. You can include a reference to a project in our Subversion repository in your SVN hosted project and it will be automatically checked out and updated along with your code. This way you don't have to copy of the sources yourself.