Did they have an obligation to fix? Depends on obligation to who.
Customers? Not really - a customer was not forced to buy one, and they could return it.
Shareholders? Possibly, as the value of the company decreases with massive negative publicity and a lack of a "fix". But then you have to ask yourself how much of a day to day obligation do companies have to their shareholders?
I would argue that while there was no obligation to fix it, there should have been a strong motivation to either fix it, or offer some alternative solution (which they did, but only after it seemed like lots of pushing).
Let me put it this way - with the next iPhone release, will their numbers of pre-orders (and people waiting in line or release day) be affected? If 10% of the people who normally WOULD have waited in line or pre-ordered decide to wait and see how well the new phone works, that could affect their quarterly reports, not to mention opportunities for rival companies to entire them away with another phone. An undecided customer is a potentially lost customer.
Chris