I've actually been looking for this info for a while. Firstly, in the US: no. In the rest of the world it is. For instance, during the earthquake/sunami in southern asia a few years back cell broadcasting was used to give out info on where relief was being given, where shelters were available and where the red cross was giving out supplies.
Its a great concept, the towers in a specified area can alert any user on that tower about site specific issues: storms, terrorist attacks, amber alerts, etc. Channels can be given out to airports to send out what the daily security level is when you are inside the airport. A city can inform people of potential detours when major road work is going to be done. All sorts of ideas are possible and since you select which channels you want to "subscribe" to, you decide what info you'd receive. Sadly though it has not been adopted in the US, with GSM coming in 2nd place for coverage and # users. I have mine enabled with some of the std channels enabled in the hopes that some random tower in the US has it enabled and I get to try it out.
Posted from my CrackBerry at wapforums.crackberry.com