Lake Superior last froze over in 2003. It has now, again, frozen over. The frequency of freeze overs has historically been around once every 20 years. Now, in the last decade, we have seen two freeze overs.
The picture below is a beautiful satellite photo of Lake Superior from yesterday. With the well below freezing temperatures seen over the region Thursday night (-20 F), any isolated open water could have frozen.
The NWS in Marquette MI writes:
Due to the recent cold spell and below normal temperatures for much of the winter of 2008-2009, ice covers nearly all of Lake Superior. Only small areas of open water remain. This image was taken on Tuesday, March 3rd. If arctic air does not return in the next couple of weeks, it is likely that this will be the day of maximum ice cover on Lake Superior for this winter as warmer weather and periods of stronger winds through the end of this week will cause open water areas to expand. Click on the image to view a higher resolution satellite picture (image is large — just under 1mb).