If you hold down the option key while resizing a "new-style" open panel that uses column view, the panel will switch to a single column mode that somewhat emulates Classic open panels (except that you navigate up the filesystem by using a horizontal scroller instead of the pop-up menu at the top of the window).
I think this just works for Cocoa apps, but I haven't checked any Carbon apps to be sure. I also can't figure out if this is a bug or a feature...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20001222232352963