A few minutes and Google searches later, I discovered that the statistic was taken from a July 27, 2009 paper from the VirginiaTech Transportation Institute.
"Several large‐scale, naturalistic driving studies (using sophisticated cameras and instrumentation in participants’ personal vehicles) conducted by the Virginia Tech Transportation Institute (VTTI), provide a clear picture of driver distraction and cell phone use under real‐world driving conditions. Combined, these studies continuously observed drivers for more than 6 million miles of driving."
In the provided table was the statistic that was reported in MacWorld: Text messaging - 23.2 times as high as non‐distracted driving.
The bottom line on this one: DON'T DO IT!!
You can read the report that I found on the VTTI website (includes a PDF download link) or you can read an article that was published in the New York Times.