The Twitter co-founder's credit card service Square, has announced that it will be selling its Square credit card readers in Walmart - expanding the product outside of tech-based shopping outlets.

Previously the Square card reader was only available from shops such as Best Buy, Radioshack and Apple stores, and this move to the main stream increases the number of outlets the device is sold to over 9000.

The Square card reader bolts onto your iPhone or Android smartphone/tablet via the headphone jack, and then, by way of an app, enables retailers to charge for services by asking customers to swipe their cards through the device. This method is far cheaper than the regular terminals we're used to, and may well appeal to traders who run businesses outdoors, such as market stalls owners - giving customers greater flexibility as to where they shop. Square then creams off the top, by taking 2.75 per cent of the transaction.

The issue is that it rather flies in the face of near field communication (NFC) which enables users of smartphones to pay for goods without the need for a credit card, but whilst the infrastructure for NFC is slow to be implemented it could leave a window for the Square form of payment. There is still, however, the issue of having decent 3G reception, if not it could lead to very grumpy customers.

For those in the UK, the move to start selling the $10 devices in Walmart means, if they sell well, we could see them appearing in greater numbers over here as UK Asda stores are now a subsidiary of the US supermarket chain.

What do you think of the tech? Does it have a future?  Let us know in the comments...