Paul, I understand your argument about efficiency. Of course a full charge from the house is better than running the engine for 30 minutes to charge the battery.
But without battery assist, this car will be a dog. On my 41st mile, I won't be able to pass, go up hills or have any fun whatsoever.
If I were to write the software (which I do for a living) I'd definately have driver options. Maybe a charge minimum setting, below which the engine starts a major charge. Another setting that indicates you're almost home, so no need to charge any further.
I don't think the car will do well in reviews if it is only a viable car for 40 (maybe) miles.
The silver one was attracctive, what's that red bland mobile?
I believe Paul Niedermeyer's answer is indeed the correct one, plus the Volt only uses half of its total battery charge -- from 30-80%. When the initial 40 miles (or so) are complete and the engine fires up, the battery is not discharged completely and will still act as a buffer to provide extra juice for passing or climbing a hill.
In sum, I don't think that the ICE will fully charge the battery pack, but does keep a certain amount of charge flowing in to maintain performance and longevity.