The UK newspaper The Independent has a story about a "limited" study of the effect of mobile phones on bees. Scientists tried placing mobile handsets near beehives and found that bees will not return to a hive with a handset nearby. The study's results are anecdotal at this time, but the lead scientist says this is a tantalizing "hint" of what may be causing the colony collapse.
These results should be taken with a grain of salt, however. Since European countries are more compact, their mobile phone networks tend to be more pervasive than in the United States, which makes me wonder why - as the article states - CCD has not affected the UK yet. It would, however, explain why CCD has primarily affected the Eastern United States.
Update 5/2: This intriguing hypothesis is apparently complete bubkus. My apologies!