I've been having this problem since upgrading to iOS 6 on my iPhone 4s. When I compose a new text message to someone with an iPhone with iMessage activated, it doesn't switch to iMessage automatically and sent it as a text message but when they reply, it's in iMessage. I'm assuming there is something wrong on my end. It happens all the time when I compose a new message to other iMessage users. My settings seem fine. Anyone else experiencing this issue?