From http://www.kde.org/stuff/clipart.php:
Trademark License
Copying of the KDE Logo is subject to the LGPL copyright license.
Trading and branding with the KDE Logo is subject to our trademark licence:
1. The KDE logo can be used freely as long as it is not used to refer to
   products other than KDE itself.
2. There is no formal procedure for using the KDE logo as long as you stay
   within the above guideline for usage.
3. Whilst not required you should acknowledge the KDE e.V.s rights by mentioning
   "KDE, K Desktop Environment and the KDE Logo are trademarks of KDE e.V."