Sans-hardware (not supported until 3.0), there are a lot of apps for the iPhone that do credit card processing, including the one that's on Apple TV ad right now: http://innerfence.com
Small retailers will snap this up and avoid the evil merchant companies. All of the inventory management and reporting lives in the cloud. Right now, besides opentable, all of the POS stuff is either really expensive or garbage.
The only comparable thing right now is a Symbol MC70 with Windows Mobile and an external swiper. Apple retail uses them and they are $3k+ a pop. You can get some knock of Pidion 1300s for $1200. All the software is "enterprise" garbage.
This device will be given away at cost. Pair it with an ipod touch and small business climb on board in mass. A whole new slew of small business, babysitters, and freelancers will accept credit cards.
They cleanup in merchant fees at the very least, and quite possibly create the holy grail of mobile-to-mobile payments -- all nicely integrated into twitter.