If you run a business where that has you going out to or meeting your clients, then here’s a relatively inexpensive way to collect payment immediately via credit card and your iPhone. PAYware Mobile (from Verifone) is a combination hardware and iPhone app that lets you swipe credit cards.
The hardware sleeve attaches to your iPhone 3G/3GS and lets you swipe credit cards at the side, just like you would at a store. The app itself lets the buyer sign their name (stylus included), and transmit the payment information with end-to-end encryption.
Of course, as with any other system that lets you accept credit cards, you do need a merchant account. PAYware itself is free with a 2-year agreement, and a $49 activation fee. Merchant account fees are additional and separate.
PS> Hardware and apps are planned for other platforms (such as Blackberry, Android and Windows Mobile.