Microbiologists in England have done some research that shows mobile phones can easily collect more bacteria than a toilet seat or door handle. They point to the constant handling (and contact with the face) combined with heat generated by the phone.
They include Staphylococcus aureus, which can cause illnesses from pimples and boils to pneumonia and meningitis and is a close relative of the superbug MRSA.
Anti-bacterial wipes should help clean phones. Unsurprisingly Korea beat the rest of the world to the punch and started shipping an anti-bacterial coated phone over a year ago. Perhaps the trend will continue.