Well, this has been coming longer than iPhone on Verizon (NYT says 2011 and has moved from #3 to #2 most emailed at nyt.com in the time this post was written).
For us at MID, it will save time of inputing the card numbers manually. (No lie: we totally thought of this while at the 2009 AMI refresher course. Alas.) There are competitors for Square and we know they're not far behind. Look for healthy competition at an AMI Refresher Course near you (if you live in the LBC). Off the cuff, I'd say that little square better be robust, because it looks like it could just snap off leaving the headphone jack hosed up.
