QGIS » QGIS Application

One of the startup tips says this (src\\app\\qgstipfactory.cpp):

myTip.setContent( tr( "Quantum GIS is open source software."
" This means that the software source code can be freely viewed"
" and modified. The GPL places a restriction that any modifications"
" you make must be made available to the Quantum GIS project, and"
" that you can not create a new version of Quantum GIS under a"
" 'closed source' license. Visit <a href=\\"http://qgis.org\\">"
" the QGIS home page (http://qgis.org)</a> for more"
" information." ) );

The GPL doesn't actually say that you need to make any changes available to the project, just that if anyone else receives your modified version then you need to make the source changes available to them.
Yes, it is preferable for people to contribute their changes back to the project rather than keeping them to themselves. But it would probably be best not to spread misinformation by saying that they're required to...