We’ve covered mobile payment startup Square in the past, but word that the device will soon be available at Walmarts across the country shows how the device is moving into the mainstream.

According to Bloomberg, Square will soon be available at the world’s largest retailer, which operates more than 3,800 stores in the U.S. Invented by Jack Dorsey, who also brought the world Twitter, Square is a stamp-sized credit card reader that attaches to iPhones, iPads and Android devices through the headphone jack.

Previously, Square was available only at select Apple retail stores, Target, RadioShack and Best Buy for about $10 (free on the Web). Square makes its money by taking a 2.75 percent cut of all transactions, a percentage that’s less than what credit card companies typically charge businesses.

Mobile payment could be a huge boon for small businesses that have mobile technicians out in the field. By reducing barriers to credit card transactions, and by shortening the payment cycle, billing and payment – often a major hassle for small businesses – could become much easier.

According to the Bloomberg report, Square processes more than $2 billion in transactions every year and has more than 800,000 merchants using the service – numbers that sure to grow as smartphone use becomes more widespread. Square’s popping up in small businesses all over the country. Here’s a great map from The Atlantic that shows just how widespread – both in terms of geography and industry – Square has become.

Square, and other mobile payment solutions such as Intuit, will likely compete with still-nascent near field communications technology, which allows consumers to use their phones wireless connection to make a payment. It’s Betamax vs. VHS and HD-DVD vs. Blu-Ray all over again, but regardless of which format ultimately wins out, field service firms will surely be the winners with lower credit card transaction fees and immediate payment.

More on mobile payment on The SmartVan