Okay, this doesn't happen every day (thank goodness), but let's suppose that you've got an important message, for all of your students, and which is time critical. It may represent part of your contingency planning, or it may be that there are certain key notices which you really want to reach all students now - that may be different for you, but what about something like the publication of the exam timetable? If you had to, could you get a message out? Onto the device in your students' pockets?
Universities and students can opt in to be able to distribute/receive alerts directly to their mobile phone, via text messages. These can be sporting or club venue announcements, notifications to pay fees or retrieve exam results, changes to lecture or tutorial times, as well as other time-critical news. Purdue University in the States, are just in the process of enabling a system, for their contingency planning.
There are a number of ways of achieving this, but if you are using the Windows Live @ Edu system, then you can use the Live Alerts - students can elect to receive alerts via Instant Messenger, their university email address, on their mobile phone, or via another email address (such as their Hotmail address). Oh, and if there's a cost (like SMS messages) the receiver pays - not you!
There's more on Windows Live @ Edu here, and more on Live Alerts here