iphone restore... SIM card not recognized

Hi,
I've been having a problem having to reset my iPhone lately. I've had to do it several times within the past week. I've become custom to plugging it into my iTunes and resetting it.

Yesterday my phone did this again so I plugged it into iTunes to reset it but this time iTunes claimed that it does not recognize the SIM card and refuses to reset it, leaving me without a phone.

I had the phone assessed at a Mac store last weekend and they said that there were no problems with the phone and that I would need to reset it every now and then. Tried to go to AT&T this morning but they are closed on Sundays.

Has this happened to anyone? Any suggestions?

Thanks in advance.