Not cheap, but I'm using the credit card software and merchant account through
Accept Credit Cards Anywhere on Palm, Pocket PC, Nextel, Blackberry, or PC / Laptop Swiped Rates
Problem is, you need a data connection on the phone which is $25-50/month plus the phone plus the merchant fees.
If you have a laptop, you could always get a wireless internet card and use a pc based credit card program.