My most recent experience has me leaning towards spending the time to wireframe though. Ryan warns against this as a waste of time "building higher fidelity sketches" (than paper), but to me building a sketch in omnigraffle is much faster than on paper, and it can be easier to convey more complicated user interactions as you can copy and paste etc. Also, for more interactive components of a UI, showing interactions through a wireframe is much much faster than prototyping it in javascript, so it becomes worth it to quickly validate that the interactive UI makes sense (by sharing the wireframes with a few people) before diving into the code (the simple example he had wouldn't really warrant a more interactive UI, but imagine, say, designing mint.com's transaction browsing / management / tagging app). That process might last a few extra hours, but if it saves you from building a snazzy UI that doesn't make sense, time well spent.