I'm using Handcent, but I get the same thing if I use the inbuilt messages app. Basically I select a group to send an SMS to and when I send the message it comes back that the multiple messages haven't been sent, but they actually have. First time it happened I kept trying to re-send, got the delivery error every time, despite all 5 lots of messages being sent - I spammed a load of friends! Oddly sometimes it works fine and Handcent does the correct sending of each msg one after another. Anyone had the same thing?