This is from the US FWS site
Basically, they say it should be on the threatened list, but there are too many other species that need to be protected (threatened?) first. They were petitioned from the National Wildlife Federation to protect the little "doggies" everywhere.
Luckily CDOW hasn't gone along with this Federal nonsense yet - you can still hunt all the prairie "dogs" you want - all year long.
Out here in Colorado it's pretty clear what they are heading for - stopping suburban development. Just recently they found a little mouse that is also "threatened" along the Front Range.