Regardless of what is at fault and who's responsible for designing the charger or the battery, it's Dell's responsibility to test their products and identify any weaknesses or possible dangers to the end user. It would be unreasonable to expect a design team to account for all possibilities and for-see all potential problems, but I don't think it's unreasonable to expect a company to come clean about any problems they identify after a unit has gone on sale and deal with them as quickly as possible.
From what I read, Dell have only publicly admitted to having a problem with these laptops after they were forced to by the media getting hold of quite a drastic example of one catching fire. If that's accurate, then I think they should have come clean about these problems straight away, before anyone was put in danger.
Brian