Here's another view of the recent cold weather: the daily mean temperature plotted against the 1981-2010 normals; grey shading of plus/minus one standard deviation.
Here are standardized daily departures for 2011. Looked at this way, the recent cold snap was about as far from normal as the hot weather in late May. The coldest day, the 17th, with an average temperature of -35F, was 2.7 standard deviations below normal, almost the same as warmest day in, when the 28th was 2.9 standard deviations above normal. The height of cold snap was a couple days longer than the hot spell in May, but the total "excessive" (the area under the curve) will likely be somewhat greater in May than in the cold snap.