AT&T is preparing to raise its rates on SMS messages to $.20 per message beginning March 30, according to a new report. The per-message raise would affect iPhone users who have gone over the 200 monthly messages afforded them in a standard iPhone voice plan. MMS multimedia messages will also see an increase to $.30 per message; the iPhone currently does not offer MMS support. In addition to the SMS rate change, the carrier has also added a new unlimited rate plan for iPhone. The plan, which runs $119.99 a month, includes unlimited voice and data, but still offers only 200 SMS messages a month. It is available now through AT&T.