In a study just published by Britain's Which? Computing magazine, alarming levels of dangerous bacteria and other gross-out biological matter was found on many computer keyboards. A microbiologist examined and measured bacteria levels on 33 keyboards in an office, and one was found to be five times dirtier than the office toilet seat, carrying 150 times the acceptable limit of bacteria. Two others had 'warning' levels of bacteria, and a further two had elevated levels of coliform, which is usually associated with fecal matter.
Eating at your desk along with poor personal hygiene are primarily responsible for these problems, so it might make sense to invest in some kind of cleaning device. Some have also said that you can actually wash a standard keyboard in your dishwasher as long as you remove it before it melts in the dry cycle, but don't take my word for it.