It happened today, and their voice and texts were actually not affected - only the email. They're speculating that it was due to a software upgrade, but I haven't heard a definite cause of why it happened. ?
I suppose because all Blackberry email goes through the RIM servers, it is possible for ALL customers of that service to go down. As someone already stated, it would only affect email and not voice or SMS services, as those go through the carriers. Blackberry service goes through the carrier's data service to the RIM data center and then out to the registered devices.