If you’ve got phone numbers stored with your contacts then if they go offline when you’re talking to them, your messages will be sent as SMS instead. Best of all, it’ll give the person you SMS a number that they can email in the future to get messages to your GMail account.
The functionality hasn’t arrived over this side of the pond yet, but it’s a lovely little bonus feature, so I hope it shows up soon. It’s yet another reason why you should use GMail over Yahoo or Hotmail.
GMail (via GoogleWatch)