i have noticed this too, unlike the other non-helpful comments the message does appear blue and sends as imessage, but then what happens is the sending fails for whatever reason and defaults to SMS (green) without warning, even though it was sent as a blue imessage.
I have verizon so it might be a data/voice at the same time thing. The last time it happened i was updating an app and then replied to a text, it was sent as sms since the app was using the data at the moment. Not a big deal but it would be nice to have a warning before it send as sms in case people have no texting plan at all to avoid .25 or whatever a pop