When I fast came across Windows 7 taskbar combined with "quick start" toolbar, I thought it's an ugly idea. But today I noticed that I tend to move my applications into the system tray anyway - as an example, take a music player - I usually click on the tray icon, switch the current track and click on the tray icon again to hide the window - on my KDE I could even configure it so that it doesn't pollute my taskbar during this operation, which is quite convenient. I also noticed I do the same thing with other applications - for example the mail client and IM.
If I was to tell what's the difference between the taskbar and system tray, I'd say that system tray is used for long-term applications that generally work in the background and signal their state via their system tray icon, as opposed to the classic taskbar, which uses the window title for this purpose (which isn't really true anymore in Windows 7).
So, here's my question - is there any valid reason to split the traditional toolbar at the bottom of the screen into quick start, taskbar and system tray? With Windows 7 basically removing the quick start bar, would merging the taskbar and system tray the next valid step?