Sharing some insight into connectivity messages for mobile applications. Based on some recent ethnography done my myself, and prompted by a real business case, I would recommend a message that:

• In plain language, briefly and directly tells the user what is wrong and why. Something like: Cannot connect because of a network problem.
  
• Affords the user a means to retry connecting (or attempts automatically). Mobile context of use means users use anticipate interruptibility and disruption of task, so they will try again as an effective course of action.
  
• Tells the user when connection is re-established, and off they go.
  
• Saves any work already done, implicitly. (Bonus points on the ADF critical task setting scale)
  

The following images showing my experience reading ADF-EMG Google Groups notification my (Android ICS) Samsung Galaxy S2 during a loss of WiFi give you a good idea of a suitable kind of messaging user experience for mobile apps in this kind of scenario.

The UX possible is dependent on device and platform features, sure, so remember to integrate with the device capability (see point 10 of this great article on mobile design by Brent White and Lynn Hnilo-Rampoldi) but taking these considerations into account is far superior to a context-free dumbed down common error message repurposed from the desktop mentality about the connection to the server being lost, so just "Click OK" or "Contact your sysadmin.".