Did you call AT&T, sometimes they can reset or resend signals to your phone. I went in for not receiving calls just receiving voicemails. They changed my sim card and resent signals and its been fine since. If not you may need your iPhone replaced.
Well I figured it out after four calls to AT&T and three weeks without sms.
Long story short, about five weeks ago I asked AT&T to give me a new phone number. They did and my sms worked for about a week or so, then it stopped working all together.
Apparently AT&T recycles phone numbers and the one they gave me had previously belonged to a US Cellular customer. Well, that number had a restriction on sms. Not sure why it worked for a week or so before it crapped out..
I could go on and on about the multiple iterations of failed correction attempts, but I'll just say I did come out $55 richer in the end.
So all in all, I'd imagine it's a pretty rare event, BUT I thought I'd write just incase someone else is going though the same runaround I did. This COULD be your problem, too.
iPhone3gs new connection had for a week everything fine but cannot receive international texts , my family receives my texts but i never get reply, so far they "att" have replaced sim still no go and we a week in with several trobleshooting calls and tech call still not solved