If anyone says use the ISP settings (oppose WAP), please don't; those are for laptop connect customers and you may get charged data $.10 per KB. Also everything is case-sensitive.
No they don't bill you extra; just don't use more than 5GB of bandwidth a month; if you do they will flag your account and send you letter. I've been doing it for years, switching my SIM card into a data card; recently I got a data connect plan because you can't get calls on the data card...
Also for the data card normally the sellings are isp.cingular, ISP@...; don't use those; they are bit faster, but they are reserved for data connect customers.