Google started showing search refinements in mid 2006. Later on that year, they cleaned up those refinements and they began showing up a lot for medical searches and for those that participated in the Google Coop solution.
But now, a WebmasterWorld thread reports that Google is showing less and less refinements these days, then they have in the past. In fact, searches that worked in the past to trigger then, don't all work. One example is clinical trials, and also site:www.seroundtable.com florida does not work. You can see that it does work if you force the more condition to be added to the search parameter.
Google frequently changes how the search results interact with searchers. Maybe the refinements were not used all that much or maybe they confused searchers. Or maybe them not showing up as often, recently, might be a bug? We don't know.
Forum discussion at WebmasterWorld.