Would you mind adding a check to ensure the `should` ran?  I should have added this when I wrote this.  Something like:

    filter_was_run = false
    WorkspacePresenter.filter(:owned_by_bob) do |scope, string|
      filter_was_run = true
      string.should be_a(String)
      scope
    end
    result = @presenter_collection.presenting("workspaces", :params => { :owned_by_bob => { :wut => "is this?" } }) { Workspace.where(nil) }
    filter_was_run.should be_true