Usually, when taskbar grouping is enabled, clicking the button on XP/Vista would show a menu where you can choose which individual window to bring up.
On Win7 the menu is shown both on hover and on click.
What I would rather see is that all of the windows belonging to that taskbar button/group/application were brought to front.
On Win7 this would make the interaction more mac-esque since the "superbar" already visually recalls the dock, and you would still be able to bring up single windows using the hover menu, and by ctrl-clicking the icon (window cycling);
on XP/Vista you would lose the per-window access (a ctrl-click emulation is left to the desire of the developer)
Hope you'll like this idea!