iPhone 4s not sending ichat messages.
I have had my iPhone 4S (white) for a little over a week. Previously I owned a 3GS. Last night and all the times before my phone would send iMessage messages when I text someone that has an iPhone. This morning it refuses to send them and every text I send it fails then I have to click send as text. Other people in my area, including my roommate, have AT&T and iPhone 4/4S and theirs works fine. Earlier this morning my girlfriends phone wouldn't send them either, and were on the same plan, but now hers works and mine still doesn't. Any help would be great. Has worked perfect this last week and previously on my 3GS.
Edit: Sorry realized I said iChat not iMessage in topic.
iPhone 4S, iOS 5.0.1, 16 gb White