I sent a text message to my girlfriend yesterday afternoon. Never got a reply.
It said it was delivered, but when I asked her later that evening about it she said she never got one.
We're both Verzion, and using new 4s phones. Tried rebooting both phones, never got it. Also, we've text mesg'd since, and it's been fine.
See the screen shots below. I took the screen shots 9 hours later.
If Apple couldn't send the iMessage it would revert to Text. But in this case, it says it was delivered?? So IMO this isn't a "server down" issue, it's a "never sent it, but marked it as sent anyway" issue.
Has this happened to anyone else? It was yesterday (Nov 3rd) around noon EST.