I really like static DCs, so that terms like "Easy," "Normal," "Hard" and "Extreme" are fixed DCs. Shadowdark does this and sets them at 9, 12, 15 and 18, respectively.
However, Kelsey does do a very interesting thing and notes in one place that one should "take the creature into account" when setting DCs. So a particular task might be "Hard" for one character and "Easy" for another based on their degree of skill.
I like the idea, but haven't fully figured out to implement it in game. Off the top of my head, I'm thinking something like: "The given DCs represent the base chance for a competent adventurer. A particular creature might see the difficulty shifted in either direction by 1, 2, or even 3 tiers, depending on how skilled they are. For example, a typical farmer has a harder time climbing a cliff than even a novice adventurer, but the greatest wilderness guide in the kingdom might find it easier, even if he entirely lacks class levels."