Instapundit links to an article at MIT's Technology Review with tips on keeping your cell phone running as long as possible in the aftermath of a disaster. The article has some good tips but one that is missing is to keep a spare, charged battery with you. Spare batteries are light and not very expensive. This is especially important for smartphones, since the large touch screens use up a lot of juice.
For example, after returning to work following the February 2010 Snowpocalypse, when Philadelphia got hit with two Nor'Easters in one week, I had the need to swap batteries in my Motorola Droid. When coming home from work, a railroad switch got frozen and SEPTA (Philly's mass transit provider) dumped me off at Wayne Junction in North Philly. I.e., da hood. I'd failed to fully charge my phone before leaving the office, but since I was able to swap in a spare, I was able to call for my wife to come pick me up.
Now that I'm using an iPhone, I don't have a user-replaceable battery. So, I got a Duracell Instant USB Charger with Lithium ion battery, that I keep in my laptop bag. Several other rechargeable batteries are available here.
With cell phones being such an important part of modern communications, and especially since pay phones are now rare, it's important to be able to keep them running in an emergency.