To reduce losses from mites and other pests, beekeepers starting using insecticides in hives. This increases "stress" on honeybees. In addition trucking hives about seems to add to the stress.
Bee disappearance occurs over the winter. The official name for this is Colony Collapse Disorder (CCD). U.S. spring losses for the past three years are: 2007 (31%); 2008 (36%), and 2009 (29%). Losses for the 2010 season are not yet known. Commercial beekeepers seem to incur more losses than hobbyists at a distance from urban activity.
Europeans have also experienced honeybee losses. Germany banned nicotinoid insecticides from use in both hives a fields in an effort to quell CCD. This has not been done in the United States, but the small community of professional entomologists battling this have even mapped the honeybee genome.
In Europe and Britain, Bumblebees are also in severe decline, but that population loss is not yet alarming in the United States.
To see this for yourself, just start checking how much wild clover you see growing. It's obvious reduced from 20-30-50 years ago. And to dig into this complex story, Google honeybee disappearance.