With a pruned wallet possible in master I believe we should expose the option in the GUI.

The Main tab in preferences will offer a chance to toggle the node type and size.
<img width="612" alt="pruning_gui_off" src="https://cloud.githubusercontent.com/assets/216917/8766147/834fb842-2e2c-11e5-94b0-c342132243bf.png">
<img width="612" alt="pruning_gui_on" src="https://cloud.githubusercontent.com/assets/216917/8766148/8351a0c6-2e2c-11e5-841b-af02fd77ff44.png">

The prune size UI should not permit sizes less than 550.

{
    
    QMessageBox::information(this, tr("Configuration options"),
        tr("The configuration file is used to specify advanced user options which override GUI settings. "
           "Additionally, any command-line options will override this configuration file."));
    
    if (!GUIUtil::openBitcoinConf())
        QMessageBox::critical(this, tr("Error"), tr("The configuration file could not be opened."));
}
void OptionsDialog::on_okButton_clicked()
{
    mapper->submit();
    accept();
    updateDefaultProxyNets();
}
void OptionsDialog::on_cancelButton_clicked()
{
    reject();
}
void OptionsDialog::on_hideTrayIcon_stateChanged(int fState)