Taxonomy exposed filter
Views Hacks does this with Views Selective Exposed Filters:


  Views Selective Exposed Filters
  
  On list and checkbox exposed filters, you can specify the filter
  option "Limit list to result set" to only display values that are
  actually returned in the view's results. The filter option "Further
  limit list to active filters" restricts the displayed values to view
  results obtained by applying exposed filter values.