Graphic from a new study:

The study, from the Virginia Tech Transportation Institute (see here - PDF) says:

…Text messaging, which had the highest risk of over 20 times worse than driving while not using a phone, had the longest duration of eyes off road time (4.6s over a 6 s interval).  That equates to traveling the length of a football field at 55 mph without looking at the roadway.

Pretty dangerous.