It’s all official now as T-Mobile announces “Magenta Deal Days” beginning September 21st and running through September 30th.

From Sept. 21–30, new and existing T-Mobile customers can purchase any device, including the Samsung Galaxy S® III, for $0.00 down payment on qualifying Unlimited Value™ and Unlimited Nationwide 4G Data plans after mail-in rebate card.

T-Mobile’s big sale push is supported by the recent launch of their Unlimited Nationwide 4G Data plan, which features no data caps, no speed limits and no overages. In fact, they cite a study that shows data now makes up over 85% of all the mobile traffic in the US.

New customers qualify for this deal when they sign a new two-year agreement on any qualifying Unlimited Value and Unlimited Nationwide 4G data plan, including the 5GB and 10GB plans. Existing customers are also welcome to take advantage of this offer with migration over to T-Mobile’s Value Plans.

“Our customers continue to appreciate our ‘Magenta Deal Days’ offerings because they showcase our commitment to giving all our customers the best 4G experiences,” said Andrew Sherrard, senior vice president of marketing, T-Mobile USA. “With back to school in full swing, we’re glad to offer another great deal to keep families in touch.”

The promotion requires for a down payment, so it’s technically not zero down— for example, the Samsung Galaxy S III on a Value Plan will require an out-of-pocket down payment of $149.99, with a mail-in rebate card of $150 for a net down payment post-rebate for $0.00. Technically, it’s not zero down, but rebates are an unfortunate and yet familiar part of the wireless business.

Hit the press release for all the details or jump to the T-Mobile link below and start picking out whats going in your shopping cart.

T-Mobile

Full Press Release:

                                              T-Mobile Helps Customers Fall Into Savings

From Sept. 21–30, get any device, including the Samsung Galaxy S^® III, for $0.00 down

BELLEVUE, Wash. — Sept. 18, 2012 — Prices are falling at T-Mobile^® this September during the “Magenta Deal Days” Zero Down promotion, making staying in touch more affordable than ever. Starting Sept. 21, customers can purchase any device, including the Samsung Galaxy S^® III, for $0.00 down payment on qualifying Unlimited Value^TM and Unlimited Nationwide 4G Data plans after mail-in rebate card.

“Our customers continue to appreciate our ‘Magenta Deal Days’ offerings because they showcase our commitment to giving all our customers the best 4G experiences,” said Andrew Sherrard, senior vice president of marketing, T-Mobile USA. “With back to school in full swing, we’re glad to offer another great deal to keep families in touch.”

The Samsung Galaxy S III is one of T-Mobile’s fastest 4G (HSPA+ 42 technology) smartphones. With an intuitive design, the Galaxy S III enables customers to effortlessly share photos, videos and more to a compatible TV or other Galaxy S III devices. Featuring a beautiful 4.8-inch HD Super AMOLED™ screen, the Galaxy S III packs a powerful entertainment experience, which allows customers to watch HD movies and TV shows, download apps, and securely browse the Web.

New customers will be eligible for this deal when they sign a two-year agreement on any qualifying Unlimited Value and Unlimited Nationwide 4G Data plan (Unlimited, 5GB or 10GB). Existing customers also can take advantage of this offer.¹

Data-hungry smartphone customers can choose the ultimate worry-free experience with T-Mobile’s new Unlimited Nationwide 4G Data plan, featuring no data caps, no speed limits and no overages, as well as fast, dependable nationwide 4G coverage. For more details about T-Mobile’s Unlimited Nationwide 4G Data plan, visit http://www.T-Mobile.com/unlimited-data-plan.

The following is a sample of device pricing and EIP payments included in the “Magenta Deal Days” offer with a qualifying Unlimited Value plan:

The “Magenta Deal Days” promotion will be available at participating T-Mobile retail stores nationwide. For more information about T-Mobile’s “Magenta Deal Days” promotion and details about qualifying plans and services, visit http://bit.ly/NldCS8.

¹ Standard migration fees will apply for existing Classic customers who switch to a Value plan and have less than 18 months of contract tenure.

² Equipment installment plans: On approved credit and 0 percent APR. Down payment and unfinanced portion required at purchase. Remaining balance paid in 20 monthly installments.