OK, thanks Fiona. Here's some more info as things are never cut and dry and even reading more on this today I learnt a few new points (be they theory or not).

Dependent lung—the lung in the lowest part of the gravitational field, i.e., the base when in the upright position; the dorsal portion when supine.

The dependent and non- dependent lung is not really important with postural drainage: the lung with secretions should be uppermost.
Why put it uppermost? Gravity yes, but also to increase alveolar perfusion to the dependent segment so as to decrease any hypoxic stress to the patient? In a general sense I feel there is some thought process that considers the dependent lung in postural drainage other than just gravity. The non-dependent lung, in the case of a patient with widespread secretions/infection etc would be breathing at a lower tidal volume. In this case I thin the ventilation is preferentially distributed to the non-dependent lung? Also I read that "in the anaesthetised patient, however, irrespective of the mode of ventilation, the upper lung receives more gas flow." So one might actually have to consider the implications of the definition as it relates to the actual patient situation.

Regional Variation in Lung Ventilation
Ventilation within the lungs is greatest near the bases, in the upright position. This is probably mainly due to variation in intra-pleural pressure as we move from apex to base - pressure is more negative near the apex. Effectively, this probably causes more expansion of the apices at FRC. As the person inhales, it is easier to expand the bases, as these are less distended than the apices! Note that at lower lung volumes, the situation may be reversed, with poor ventilation of the bases.
A few more notes

Ventilation-perfusion mismatch

This is an important cause of hypoxaemia affecting patients on the intensive care unit. Relative ventilation and perfusion, in different areas of the lung are unequal, resulting in inefficient gas transfer.

The distribution of ventilation in normal subjects varies depending on the mode of ventilation and position. More ventilation occurs in the right lung due to its larger size, both in the upright and supine positions. In the upright position there is greater ventilation towards the apex of the lungs than to the bases. In the lateral position, the lower lung is preferentially ventilated irrespective of which side is lain upon. This is due to the dependent diaphragm lying higher in the thorax, with increased length of muscle fibres providing more efficient contraction during inspiration. In the anaesthetised patient, however, irrespective of the mode of ventilation, the upper lung receives more gas flow.

Pulmonary blood flow is greater at the bases than at the apices in the erect subject. The distribution of flow through the lung is uneven due to the relatively low pressures in the pulmonary circulation and gravity assumes a very important role. Similarly, in the lateral position, the dependent lung is perfused more than the upper lung.

Although both perfusion and ventilation increase from the apices to the bases the increase in ventilation is less than that of perfusion and, in order to understand the relationship between the two is described as the ventilation/perfusion ratio (V/Q). The resting values are approximately 4 l/min for ventilation and 5 l/min for pulmonary blood flow, giving a ratio of 0.8 throughout the whole lung (assuming ventilation and perfusion of all alveoli are equal). However, some alveoli will receive no ventilation and some will receive no perfusion.

Hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a potent regulator of the distribution of blood flow to match areas of ventilation. HPV normally acts to improve gas exchange by reducing the blood flow to lung regions with low V/Q ratios.

In conditions producing inflammatory mediators such as sepsis and trauma, HPV is impaired with blood flowing to poorly ventilated lung resulting in hypoxia. Drugs such as sodium nitroprusside and nitroglycerine can also impair HPV. Hypoxic pulmonary vasoconstriction can also be abolished in the presence of raised pulmonary artery pressures leading to V/Q mismatch and hypoxia.

Ventilation perfusion mismatch is responsible for the hypoxaemia seen in pulmonary oedema, chronic obstructive airways disease, pulmonary embolism and interstitial lung disease. The hypoxaemia worsens with increasing V/Q mismatch for two reasons. Firstly, with V/Q mismatch, a greater percentage of the cardiac output passes through lung units with lower V/Q ratios (perfusion > ventilation) so that less well saturated blood makes a greater contribution to total pulmonary blood flow. Secondly, in relation to shunts, the oxygen content of blood from lung units with low V/Q ratios exerts a greater effect on the saturation of blood flowing to the left side of the circulation because of the shape of the oxygen dissociation curve.