Actually, I was thinking of systemic re-sends - as I understand it, Blackberries at least - and I assume something similar applies to SMS - have a 'message received' acknowledgment. If the first packet doesn't get through, then the automatic re-send probably will, and it just keeps trying until it does. Much better than voice, where a missed data packet is gone to the bit bucket forever, and you have a signal degradation of some kind.