iMessage says 'Delivered' but actually hasn't

Along with having extremely poor battery life and echoing when I make calls with the headset on my new iPhone 4S, I am also experiencing trouble with iMessage. For the most part it works and sends texts/iMessages and delivers them fine - but I've found that on three separate occasions (to three different people) that although iMessage says it has 'Delivered' the message (it's a blue bubble), none of the recipients have received the message. There doesn't seem to be any correlation between the three people - two have 3GS with iOS5, one has a 4 with iOS5. I have messaged other people with a 3GS and a 4 without any problem at all.

Not sure what steps to take now... any ideas?

Thanks in advance!