I just came across a blog posting on Tech-Recipe about grouping application windows in Ubuntu/GNOME taskbar. It's a useful optimization for newbies as well as the old-timers running a GNOME-based desktop. This feature allows a user to group application windows together in a way similar to Windows XP.
The way to enable application window grouping is simple. On my desktop I right click on the dotted vertical line between the quickstart icons and my open application windows. With my setup this is on the bottom taskbar (which is default with GNOME on Ubuntu). Select Preferences. You'll see a window similar to the image here. Under Windows Grouping yo have three choices; Never group windows (default), Group windows when space is limited, and always group windows.
[Enable Window Grouping on the Window List] -- Tech-Recipes Blog