The iPhone app and the card reader are free... however every transaction processed has a fee (15 cents + 2.75% of the transaction, if the card is swiped. More if you type the number in) that is charged. So if you take it in and get someone to donate $100 through it, you'll only actually get $97.10. Not a huge hit, but something to consider.
A better option is to utilize one of many publicly available fund-raising sites that don't charge processing fees. A great example the Robettes have been using is
GiveMN. We set up our own fund-raising page there, and people can donate there any time they want. We can direct them there from our website, or from any promotional materials we can leave for businesses to consider. Most businesses have internet access available and we can help them with the donation while visiting them, or bring along a laptop/iPhone/Android/whatever they can donate through. Of course, that site is specific to Minnesota, but there are other similar sites out there.