The entire serial/parallel hybrid decision depends on whether or not you believe that battery technology will improve and become practical for mass-market use. That's the big question.
Toyota, Ford, and GM invested a lot in parallel hybrids when the belief was that batteries couldn't make the big jump. Toyota and Ford increase mileage by 20%-40% with only a 1.6KHW NMH battery, and GM dramatically improves an Escalade with only 2.3KWH. If all you have is about 2KWH to work with (and only half of that practically usable), you need the parallel hybrid to make the car go because the battery can't get you to 60MPH without the engine's help, and it's not worth paying for (and carrying around) a charging module to add a useful 1KWH to the serial hybrid battery. That's why the Prius plug-in is only going to be a publicity vehicle, and not sent out in wide use.
However, if you have more to work with, the rules of the game change. If you have 16KWH and about 150KW to work with, you can build a serial hybrid like the Volt or Karma, simplify the drive train, and let the battery charge a useful 10KWH or so from the wall outlet. If that's the case, then a serial plug-in hybrid makes sense. If you can hold even more, like about 60KWH, then even the range-extender goes away and you have an all-electric vehicle.
Even as someone not involved with car engineering, it seems logical to me that the design of the car will follow the battery. Until recently, 2KWH was the practical size from a cost and technology basis, and the parallel hybrid was the technology to beat. With GM able to get 16KWH batteries for $6000, it's probable that serial hybrids are the upcoming trend (assuming we don't run out of lithium). The parallel hybrids will be around for many years because of sunk engineering costs at Toyota and Ford, but they'll go to serial hybrids eventually. If, however, someone generates a breakthrough and a reasonably-priced 60KWH battery becomes available, it's game over and the pure-electrics will dominate. Tesla's betting the company on that outcome.
In the end, watch the batteries. Everything follows that.