New York Times has recently published the result of a study that "Texting Lifts Crash Risk by Large Margin" - the collision risk was 23 times greater than when not texting.
In Singapore, Section 65B of the Road Traffic Act covers texting while driving - as long as there is evidence to show that the motorist was holding onto the handphone with one hand, while using it to communicate. This is regardless of the position in which he was holding onto the handphone and the fact that a hands-free kit was also used is irrelevant. The various scenarios are clearly described:
Scenario B (SMS/MMS message)
Writing, Sending, and Reading SMS/MMS messages from someone, with or without earpiece, using one hand to hold on to the mobile phone and the other hand holding the steering wheel.
If you receive a text message while you are driving, do resist the urge to reply immediately especially when your vehicle is still in motion. Remember that the sender will call if it is an urgent matter. Drive safely.