I have only had my 2G about a month and have not received anything from AT&T saying they will add a data plan automatically, although they know I have the iPhone because it shows up in my online account.
That doesn't mean you won't be caught in a sweep later. The OLAM display is more or less automatic, but there doesn't seem to be much rhyme or reason to when they conduct TAC audits or who they do them for. You might luck out, you might not.