I just don't see the point in this, but I don't see the point of tap and go either.
With Tap and go, you still need to wait for the store to scan the product etc, does it really save that much time? Why don't they just adapt the current swipe units to not require pin or signature for small amounts, so it would be swipe and go, no need for another device to hang off the cash register. It already works when you pay for parking, for significant amounts to.
As for the use on the iPhone to walk up to any computer - this is going to be a niche market product at best (how many places do you know have Macs just laying around for you to walk up and use, other than Apple Stores themselves?). Much as I hate to admit it, when you look at the figures desktops are on the way out, so in a world where everyone is moving to laptops, and Apple is blazing the trail in really small, really light laptops, what is the point of a technology to allow you walk up to a computer and make it yours?
I think by the time this becomes wide spread enough to be useful iPads & Airs will be powerful and full featured enough to take the place of the computer anyway (for any task this technology could reasonably support).
Thinking further - if everything is going to live in the cloud anyway, wouldn't just logging in to an enhanced version of mobile me be able to achieve the same result? Some option to get the computer to boot from the network, and in this case the network happens to be the cloud, and your system details. It essentially sounds like a move back to thin client computing (maybe it will work this time) being over complicated by a new gadget.
Maybe as the security device to make it work - you log in through the cloud, but you need to have your iOS device with the NFC chip near the computer to actually make it work, that makes sense.
And one last thing - will the software companies re-write their EULAs to allow this type of use - most licences are currently tied to the computer, not the user, so opening it up to run on any computer I happen to be standing next to might prove interesting.