Mobile payments startup Square said today that its iPhone and Android applications are live, and that it has begun to ship its first wave of credit card readers.

The San Francisco company, started by Twitter creator Jack Dorsey, offers small plastic credit card reader that plugs into an iPhone, an iPod Touch, an iPad, or an Android phone. It first announced this technology back in December. (The origins and ownership of that credit card reading technology have been disputed.)

If you don’t have the reader, you can also just enter your credit card number into the app. Other features include email and SMS receipts, and a photo identification system for users, so other people aren’t swiping your credit card. Both the app and the reader are free.

In a letter posted today on the Square website, Dorsey writes:

Evolving something down to its essence isn’t easy (or instant). In Square’s case, it requires the identification, coordination, and navigation of many massive and forward-looking organizations. Square seeks to build an amazingly beautiful experience; every pixel, piece of plastic, rate, detail, word, policy, partner, and decision is considered, debated, and improved constantly.

Today, in addition to releasing clients for the iPhone, iPod touch, iPad, and Android devices, we are opening up our first cut at activating card processing and pricing, and sending out our first production batch of free card readers. All represent just the beginning of what Square is striving to become: a simple payment utility for everyone.

The company’s backers include Khosla Ventures, Google’s Marissa Mayer, Twitter cofounder Biz Stone, angel investor Ron Conway, Digg creator Kevin Rose, and others.

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