I work for Sprint/Nextel PCS, so I have a little insight.
It isn't just the cell towers that are a problem-a lot of the big switching stations that route land calls went through the biggest city in the area-NO. So land calls, and cell to land, and land to cell, are FUBAR. For the towers that are still up, power isn't available. Also, if a caller has their local calling area in the devastated area, the network can't authenticate their ID, even if they've left the area, and are trying to call outside the area. It's being worked on, but given the dire situation, it's going to take some time. I can't speak for the text messaging.
Sprint/Nextel also has web service on a lot of our phones, but between the chaos down there, and the fact that the Sprint customers who fled now have only their phones to access the web, a lot of people are being timed out.
It's heartbreaking to take calls from refugees.