That's normal, if you think about it - the one space is taken up by what is *normal* in the system tray - the second is taken up *only* when you have a chat window open *or* if you have your contact list open. Those, by default, always took up a taskbar slot.
So, the issue lies with the no system tray ability of WLM - and it's weird, cause I have plenty of other apps that have no issue minimizing to the system tray....