So I guess from the answers that Mathematica does not provide this facility. Here's hoping the Mathematica 9 will provide something equivalent to (or better than) the outline view in Scientific Notebook or the useful Navigation Pane in Microsoft Word or for that matter the outline view in the Wolfram Workbench. (Or, that someone will correct my impression that this functionality is currently absent in notebooks.)
Here is the best I've come up with: select the notebook title bracket and press ctrl+} to close all subgroups, and then navigate through the resulting "outline". But of course that closes all subgroups, which I do not want to do. And if I just select the cell bracket for the title and close all subgroups that way, I don't lose my open cells but I cannot navigate conveniently.