Earlier mentioned here previously, Apple's internally-developed, intended-only-for-itself POS system (to replace the handheld POS system running atop Windows Mobile, from which the Apple Store is migrating) has some external demand. Also addressing this demand is a company started by Twitter's founder, who offers a card-swiping dongle and payment support systems, on which it presumably would make a cut as a transaction middleman.
Should Apple buy the dongle-maker? I think not.
The dongle may work, but I think Apple's shell-cum-reader is slicker, easier to get in and out of holsters, and capable of being expanded better (ie, Apple could release a holster that's also a barcode scanner, or simply adapt Wil Shipley-like tech (e.g., Delicious Library) to use the camera as a bar code scanner. The choice of WiFi vs G3 connection (and iPhone v iPod Touch) in Apple's solution is nice, as it allows folks to distribute POS devices that aren't necessarily also the users' phones -- or to allow salespeople to attach the shell (while at work) to their own phone to do work -- or to deploy LAN-only iPod Touch devices.
I highly suspect that Apple's billions in credit card transactions place it in a position not only to process third parties' transactions alongside its own, but to BECOME a major transaction processor if it wants. Hmm. Transaction processors need big datacenters, don't they? Didn't Apple buy a big datacenter location?