The presentation took place at an investor meeting where Okamoto said that factoring in battery size, charge time and range, a parallel hybrid with higher capacity battery makes the most sense. Depending on how battery development progresses on cost and reliability over the next couple of years, this may well turn out to be true. However, another scenario is that battery cost will come down enough, especially if re-use is factored in, that the simplified drive-train of a series hybrid will turn out to be the more efficient and effective approach to the problem. Toyota may just be trying to downplay the prospects of series hybrids because of their own lithium battery difficulties.
[Source: Toyota, GreenCarCongress]