Seems to happen to SWMBO (Tesco). No problems for ages, then every so often a spate where the phone reports "cant send message" (with no reason). Retrying repeats the message until she gives up.
Just happened now, only (of course) when I pressed "send" the message went.
We're in the city with all bars up (not that it means much. I've had calls drop on 5 bars all the time).
The only time *I* have experienced this is in London, using the underground. Which sort of makes sense.