To download the code, use
svn co svn://arklinux.org/olpc-qt/trunk olpc-qt
and follow the instructions in the included INSTALL file.
The library code is in the lib/ subdirectory, and example activity can be found in src/.
The idea is to port an application with as little effort as possible: Just use QActivity instead of QApplication and tell QActivity what your top level window is (QActivity::setTopLevelWidget), and it will run as activity. A QActivity based application automatically detects whether or not it is being run from sugar, and runs as a normal fullscreen application if it's run outside sugar.
QActivity::dataDir(), QActivity::instanceDir() and QActivity::tmpDir() return the SUGAR_ACTIVITY_ROOT environment while running in sugar, and equivalent other directories when running outside sugar.
Currently, only the basics are implemented - help with making more features (such as better Mesh network integration) available to Qt based activities is welcome.