What shall we call these, wirethumbs or thumbframes? Once we've chosen all the ideas we'll use in our concept we try to create a sitemap and wireframes.
Sometimes I use this 'inbetween method' to make sure we didn't forget any pages in an early stage. A little bit more insightful than just a sitemap and rather quick to produce.
Update 13-11-2009: Today I came across this post on Wireframes Magazine, an idea that was originally posted by Jason Robb. Similar to what I describe here, but probably even more powerful because of the use of post-its.