Compilation of developer notes on changes in 1.6.
JDatabase::setQuery casts the sql variable to a string. This allows you to pass an object that implements the __toString magic method.
JModel::getState will now take an optional second argument to set the default. $value = $model->getState( 'foo', 'bar' );
JFile::write uses a reference for its second argument. Code like JFile::write($filename,'string'); will fail, however $data = 'string'; JFile::write($filename, $data); will work for both 1.5 and 1.6