The Census Bureau has recently released county-level population estimates for 2011. Of course, the operative word here is "estimates." Actual counting only occurs once every ten years.
As you can see from the visualization below, tiny Daggett County displayed the largest 2011growth rate (using the July 1 estimates), but added less than 100 individuals. The second fastest growing counties were neighbors--Summit and Wasatch counties. Despite its late entry to the economic recovery, Washington County managed the fourth-fastest population growth rate.
Six counties registered population declines--Emery. Carbon, Garfield, Piute, and Beaver. Interestingly, these counties form a little cluster of contracting population near the center of Utah.
You can access the Census Bureau's 2011 estimates by clicking here. The Utah Population Estimates Committee should also release their 2011 population figures in the near future.