Nextel has phones now that have a credit card reader built it, dunno what merchant service company they use, or what the rates are, but it might be somthing to look into. Maybe try talking to one of your local mobile tool salesmen, snap on, mac, matco, etc, as they have wireless credit card machines on their trucks.
Keep a close eye on rates and service charges, they will nail you for everything they can, some companys have it cheaper if you do online statments, and stuff like that, as they don't have to print off and mail your paper work to you.
I would contact your bank/credit union and see if they have a recomended service, as some banks have in house merchant services, and that always makes things a little easier. My credit card stuff is through Commercial Federal bank, same place my business checking is at.
If you have any more questions, hit me up through IM or PM or something, and I will try and answer more questions.