What is the term for the rhetorical, "How hard could it be?" I looked through the wikipedia page listing cognitive biases looking for a bias that describes the tendency to assume a poorly understood problem must be simple. Planning fallacy is kind of in the right ballpark; it gets the underestimation part, but I am associating the behaviour with a snap decision more than a plan. Is there another that fits better?

I really thought it would be easy to find this term on wikipedia. I mean, how many psychological terms could there possibly be?
I believe the closest match is the illusion of control:


  ... the tendency for people to overestimate their ability to control
  events; for example, it occurs when someone feels a sense of control
  over outcomes that they demonstrably do not influence. ... people base
  their judgments of control on "skill cues". ... When more of these
  skill cues are present, the illusion is stronger.


This bias only operates when a plausible case can be made for one's skill or intent influencing outcomes - such as a programming task given to a programmer.  In such instances, how well the task or problem is understood, its actual level of difficulty, and the actual level of control over outcomes are overestimated or ignored.

Positive illusions - biases that describe how people overestimate their own abilities, success, and control - are the broader category that contains the illusion of control, and the planning fallacy, among others.  These biases typically go away when estimating the success of others instead of one's self.

There is another bias worth noting here, called the hard-easy effect:


  ... a tendency to overestimate the probability of one's success at a
  task perceived as hard, and to underestimate the likelihood of one's
  success at a task perceived as easy. ... "Hard tasks tend to produce
  overconfidence but worse-than-average perceptions," ...


This bias also ignores how well understood a problem is, but it suggests that a problem perceived as hard (whether or not it actually is) is more likely to result in overestimating success.  This bias is not as robust or widely accepted as the former.