In the "seat bag content" thread, a few people posted that they carry their cell phones. Those who don't, why don't you? Some people say they ride to get away from phones and interruptions, but I think it's a good idea to carry it. I've used my phone 3 times while riding:

First time: Our riding group split into two smaller groups. I flatted. I was able to call a friend in "group 2" to say everything was ok, but we'd be delayed in getting to our rendezvous point. (I change my own tubes, but I'm not too quick doing it.)

Second time: I came across a group of about 6 guys on the trail. One of them had fallen. They were concerned he had neck injuries. None of the guys had their phones. None of them knew where a ranger could be found. I called 911 on my cell phone and directed the rescue team to where we were.

Third time (most important to me): Many times while I am biking, my husband is out riding his motorcycle. One day, 10 miles into a 20 mile ride (exactly at the point of no return), my phone rang. It was my husband's riding buddy. My husband had been in a serious accident. Long story, short, he fully recovered. But if I didn't have my phone with me, I wouldn't have know about the accident for several hours.

When I ride, I usually leave my phone turned off and I check my voice mail when I'm through. But when my husband is on his motorcycle, I leave it turned on.