Reflections on slime

There's lots of money being directed to research on algae as a source of biodiesel.  ExxonMobil has committed $600 million to slime research.  Closer to home, the University of Arizona has several researchers up to their elbows in algae, as described in the local newspaper.  Some strains of algae are high in lipids (fats and oils) that can be extracted and burned, and what remains is high in protein, which animals can in principle eat.  My impression is that algae may have a better return ratio than corn (1.3 to 1; lousy), but to make sense, it needs to approach the 15 to 1 return ratio of conventional oil.  I'm trying to imagine scaling to the millions of barrels (42 gal/barrel) per day necessary to provide a credible supplement to declining conventional oil.