I used to be a master at it when I had my Razr and Razr2. The physical keys allowed me to do it without even having to look at the phone at all.
When I got my Android phone, that ability severely decreased, since you really have to pay attention to the screen, so I stopped. Same with my iPhone. Now I will only text while driving if it's somewhat urgent, and I always wait until I'm stopped at a red light. Sometimes I will have to stop when the light turns green, then I just resume typing at the next light.
Also, here in Michigan, they just recently made it illegal to text while your car is in motion (which is great). I think you get a $100 fine for your first offense, and it increases each time thereafter, though I don't recall the exact details.
One time I saw this lady driving an SUV... pulled up next to her at a light and just happened to look over... somehow she was reading a book, texting on her phone, and smoking a cigarette. I don't even know how that's possible, but I wanted to punch her in the face.