I am a little curious on exactly how the low temperatures figure into the zone equation. It seems like there would be a big difference between a week where the temps were consistently cold day and night and a week where the temps dips low for just one or two nights but warmed up during the day. I am also not sure if wind chill fits into the picture.

Looking at the USDA definitions for zones I see this:
6b = -5 to 0 F
7a = 0 to 5 F
7b = 5 to 10 F

Looking a The Weather Underground (www.wunderground.com) I see that the lowest temperature recorded this year at Dulles Airport was 5 degrees on Feb 8th. The only other days with single digit lows were Feb 9th (7 degrees) and Feb 11th (8 degrees). So it seems that this "very cold" weather would still qualify as Zone 7 weather (at least for Dulles Airport).

- Brent

Here's how the current USDA Plant Hardiness Zone Map, which was done in 1990, is based.

A simple average of the lowest minimum winter temperatures each year from 1973 to 1987.

The new version, which we are working on right now, will for the first use 30 years of average minimum winter temperatures from 1973 to 2002. This should smooth out some of the year-year fluctuations.

BUT, for the first time, a sophisticated algorithm is being created that will tell the computer how to draw the zone boundries between the data points where we have actual temperatures. This algorithm will take into account elevation slope and other factors.

Of course there are many other factors that influence how a plant survives such as duration of cold, wet cold or dry cold, snow protection etc.

All plant hardiness zone maps are simply guides if for no other reason than you are asking a map based on past temperatures to predict future weather behavior. The weather people can't do that for next month let alone next year, so just how much can you expect the map to do.