With 10.4.2, it is now possible to use keyboard shortcuts to page between widgets on the Dashboard bar, instead of having to click the paging arrows on the bar itself.
Hold the Command [Apple] key down while pressing the Left Arrow (or Right Arrow) to move back (or forward) a page at a time.
[robg adds: Unfortunately, it doesn't seem as though Apple added any way to actually activate a widget via the keyboard; for that, you'll still have to grab the mouse...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050716172000226