True warming trend (of course we've known that for some time now) in addition to shifting planetary weather patterns (current shifting trend has been around for at least a few decades now). The new map is just an attempt to catch up with reality.

brandon7, you make a valid point but do you have an opinion on the plant breeding and selection aspect?

I'm kinda drawing a blank here Ronalawn. Are you asking if plant breeders will change their objectives as their climate, or as the national climate pattern, changes? If so, yes, it will probably have a small impact. Breeders may be somewhat less concerned about cold hardiness and more concerned about heat tolerance or other factors. I doubt the map, specifically, will change that though. Most of us plant people have been aware of the changes for a long time, so the map doesn't really change our plans. It can help us more quickly identify zones for corresponding places though, so, in that way, may give us a bigger, clearer picture.

brandon7, a long time ago I visited a site in Florida where they observe many varieties of bedding plants in the field. I believe that they report the 'grades' to some central site which puts it all together for use by architects, growers etc. I'm thinking that that cold hardy varieties might be identified in this way and the species might be introduced into a "lower" zone, not because the temperature there has increased, but because the new plant has been selected for its performance in the lower temperature.
I once read that there is a wider variety of trees in North America because the mountain ranges run mostly North/South and the vegetation had opportunity to adapt to the advancing ice(age). In Europe, where the mountains run East/West, the vegetation on the leeward side had no time to adapt when the ice came "over the mountain" as it were; and they were wiped out in a short time.

"I wonder if there is a true warming trend or have the seed producers (subconsciously?) selected crossings for cold hardiness?"

"or"? that should be "and".

There is no question that there is a warming trend.
AND
There is no question that breeders want to "open new markets"