If you want to have all your mounted disks/shares -- actually the content of the /Volumes folder in one stack, you have to drag the /Volumes folder onto the Stacks area from the dock, but the /Volumes folder is invisible.
However, you can reach invisible folders in the Finder with the Go -> Go to Folder menu command (Shift-Command-G). Use that menu, then enter /Volumes in the dialog box that appears. When the folder appears in the Finder, drag it to the right side of the dock to create a Volumes stack.
[robg adds: Of course, this would be much more useful if you could then navigate the pop-up drive list as you could in 10.4.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071027093531375