Nielsen has a better than usual alertbox today trying to dispel the Misconceptions about Usability. One of his points really resonated with me regarding the tensions that can exist between creativity and usability.
Design is basically problem solving under constraints: you must design a system that can actually be built, that’s within budget, and that works in the real world. Usability adds one more constraint: the system must be relatively easy for people to use. This constraint exists whether or not you include formal usability methods in your design process.
Knowing real-world facts increases creativity because it offers designers ideas about design improvement and inspires them to focus their energy on real problems.
Its hard to say that adding a constraint is going to increase creativity. However, when a designer is thinking about creativity they’re usually talking about coming up with a site design from scratch and resist the way that usability guidelines force them to work in accepted ways such as boxy-columnar layouts and avoiding mystery meat navigation. I think what he should have really said is that usability, like any other constraint, forces designers to focus their creativity to improve, mold, and refine accepted website design conventions for a particular deployment. Of course, this takes the creative joy out of the process and routinizes it.