 
You will find the version number in the file korganizer/mainwindow.cpp
method 
void MainWindow::about()

Example:
void MainWindow::about()
{
    QMessageBox::about( this, i18n("About KOrganizer/Pi"),
                        i18n("KOrganizer/Platform-independent\n") +
                        "(KO/Pi) 1.7.8 - " +

is version 1.7.8