I just read an email that claims cell phone text messaging (SMS) keeps
>going when cellular voice service is down. Please tell me if this is >true or not.
It depends on what you mean by down. The link between your phone and the tower is the same for voice and SMS, so if the tower is not operational due to an extended power loss or the backhaul link broken, then nothing works.
What's more common is an emergency overload situation. When that happens, voice calls are blocked or restricted to responder priority only, but control messages to/from the phone which includes the ability to send and receive text messages remains available.