T-Mobile is kicking off a new two-week long promotion called Magenta Deal Days that could net you $150 off of a new tablet or smartphone. You won't get the discount right away, but if you sign up for a new two-year deal with the carrier — or are an existing customer eligible for an upgrade — you'll be able to get a mail-in rebate for a select few Samsung 4G devices. The amount of the rebate depends on whether you go with a Classic or Unlimited Value plan, but the biggest deals are for the Galaxy Tab 7.0 Plus and 10.1 — both of which feature a rebate of $150. The Galaxy S Blaze 4G is also part of the promotion, as are the Exhibit II 4G and Galaxy S 4G. The deals start today and runs through to April 3rd, but you'll need to head down to your local T-Mobile location to get in on the action, as the offers are only available for in-store purchases.