My guess is that most people will soon find out that constantly being interrupted by one's cell phone makes it hard to relax or concentrate. On the other hand, it is convenient if someone is easy to reach.
Is there a way to reconcile the opposing goals of not being interrupted and being connected? Well, asynchronous communication such as email and SMS goes a long way (if one doesn't check for updates too often).
But what if you need synchronous communication? Then you can use a message to set up a time interval during which to get in touch directly. This limits the time when you have to be easily reachable.
Complementary to this approach is the idea to let people know your current status: Do you want to be contacted, are you busy etc. This idea of a publicly broadcasted status originated in instant messaging and has been further developed by the Twitter service to also work with SMS, web sites etc. I expect this kind of service to be more popular in the future (e.g. I can imagine it being better integrated with cell phones), as we grow more sensitive about being always available.