After kicking off something of an experiment in November last year to see which type of leaf cutting gave the best performance, I now have the results!
The image shows the types of cut made; i. Lateral along centre line, ii. 2-3mm cut from the bottom of the leaf, iii. Approx 10mm of stem left in tact and iv. Approx. 5mm of stem left in tact. The cuttings were made and planted out on 17 November 2012 and given two months to do their stuff. Over a two-month time span, it appears that in terms of speed and the number of plants propagated, the winner is Method IV (Approx. 5mm of stem left in tact).
However, all the methods (except II) yielded some success with Method I giving some sprouting at the stem end of the half-cut leaf. This suggests that two plants could in theory by propagated from a single leaf although given the relatively low success rate to date (approx. 60%) I would not recommend this method.
Method II gave the worst results with 0% success (although the leaves appear to still be viable).
This suggests (perhaps unsurprisingly) that the stem of the leaf is crucial to propagation and as long as this is kept moist in a suitable medium, life will arise!