The USDA recently issued a new Plant Hardiness Map that changes some of the zones.
My neighborhood in Dallas appears to have remained 8a.
It is interesting that parts of Dallas are 8b while the remainder of Dallas is 8a. In particular, parts of west Dallas fall within 8b. In fact, it appears that the zone changes less than 4 miles from my house!
Go here for an interactive map -- Interactive Map.
The new map is based upon temperature data from 1976 through 2005, and an article on NPR claims that some of the shifting zone boundaries were the result of more sophisticated mapping, taking into account for the first time the effects of elevation, large lakes, and whether a place is located in a valley or on top of a ridge.