Sometimes its not just the keyboards either. The most disgusting computers I have ever seen were in the ER area of a local hospital (where I used to work as backup to the helldesk). That said of course, some of the helpdesk techs at my current company (health insurance) are constantly complaining (with good reason) about dirty keyboards, at people's desks. Recommendation would also be to keep a pump jar (or something of that nature) of "waterless" hand sanitizer around as well.
Computers can be filthy and disgusting (and thats not even getting into the pr0n :-)