Most articles I've read indicate the likely culprit is a particular insecticide that is used pretty widely for crops, lawns and other areas. (What a shock, that. Spray crops with insecticide to kill nuisance insects, and wonder of wonders, it affects beneficial insects like honeybees too.) Commercial honeybees, the ones that get driven round and round to fertilize crops, are the ones being affected; anecdotally, the privately owned hives largely have been unaffected -- probably because if you're a honeybee hobbyist or a smalltime owner of a hive or two, you're not going to go spraying or taking the bees where people have been spraying.
France had similar problems, banned this particular insecticide, and they have seen a rebound in their honeybee populations.
Of course, insecticide might be only the chief (as opposed to sole) culprit, or one of several factors in the die-off. In any event, it makes sense to me for people to voluntarily scale back or eliminate their use of insecticides and find other ways to control pests. Organic solutions don't eradicate whole beneficial species, contaminate the ground, pollute waterways or accidentally poison children, and it's been shown that organically raised crops have a higher yield than chemically treated crops, even during drought years.
Seems a like a good approach for all of us.