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.

#ifndef BITCOIN_QT_OPTIONSDIALOG_H
#define BITCOIN_QT_OPTIONSDIALOG_H
#include <QDialog>
#include <QValidator>
class OptionsModel;
class QValidatedLineEdit;
QT_BEGIN_NAMESPACE
class QDataWidgetMapper;
QT_END_NAMESPACE
namespace Ui {
class OptionsDialog;
}
/** Proxy address widget validator, checks for a valid proxy address.
 */
class ProxyAddressValidator : public QValidator
{
    Q_OBJECT
public:
    explicit ProxyAddressValidator(QObject *parent);
    State validate(QString &input, int &pos) const;
};
class OptionsDialog : public QDialog