Everyone knows they should be flossing as well as writing their tests first. They've heard of the tools like NUnit, but they just don't do it. Take it from a guy who got drilled for several hours recently because of cavities between the teeth: it's not worth it.
On a serious note, I'm spending most of my time in my current project on bug fixing. I know that leveraging unit tests and a tool like NUnit will not only make me more productive, but also trim the amount of time I spend like today fixing bugs (all day long).
The big question is how to apply that to a web application. Many of the examples given for tests in NUnit really lend themselves to libraries (DLLs) and not to Web Forms with postback cycles.
I haven't had much time to dig into it, but pretty soon I'll have to roll up my sleeves and do a comparison of the new Visual Studio Team System test capabilities and NUnit. The NUnit people blogged a comparison but I'd like to have a look for myself. My leanings would probably be toward NUnit for simplicity, less overhead, and availability, but I'll withhold judgement until I've had a chance to get dirty.