When you create a folder for bookmarks in the Bookmarks window (Command-Option-B to open it), and put that folder on the bookmarks bar (by dragging it into the Bookmarks Bar collection), you get drop-down menu on the bookmarks bar listing those bookmarks. Change it to an “auto-click” folder, and instead you get a button that, when clicked, opens every bookmark in the folder, each in its own tab in the current window.
Safari 4’s choice for Top Sites may miss ones that you’d like included (maybe one you haven’t been to in awhile got bumped out of the lineup because you hadn’t pinned it). You don’t have to wait for it to show up on its own: