The Lifehacker blog offers up a great little registry hack for grouping similar items on your taskbar. You may have noticed that the taskbar only groups items when you have launched enough windows to fill the taskbar. However, if you want to group similar programs all the time, a simple registry hack does the trick. Go to:
Create a new DWORD (32-bit) value named TaskbarGroupSize at that location, setting the value to 2 or higher to set the number of programs to activate the grouping.
HKEY_CURRRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Explorer \ Advanced