announced the operator's ground breaking Free Data For Life (FDFL) initiative to give tablet users unlimited free Internet for life he made it sound like the offer was good for any tablet that can connect to T-Mobile's data network. It turns out that's not exactly true. T-Mobile has published a Webpage listing all the tablets eligible for the promotion. The list includes well known devices like every iPad and many Samsung Galaxy Tab variants as well as every tablet T-Mobile has sold including really some old ones like the Dell Streak 7.
But more shocking is the statement by T-Mobile the "The ASUS Nexus 7 (LTE) is excluded if it was purchased before October 31, 2013." This is especially surprising given that the new Nexus 7 was one of the tablets mentioned by Legere in his announcement. Phonenews.com is speculating that the reason pre-Nov. 1 Nexus 7's aren't supported is that they are eligible for a separate 2GB of free data for one month offer from T-Mobile. Whatever the reason, there are going to be a lot of Nexus 7 LTE users unhappy with T-Mobile.
T-Mobile also clarified the process users need to go through to enable free data on eligible tablets. Which is to go to mim.t-mobile.com using their tablet's browser and sign up.
Update 4/28/15: T-Mobile has removed the list of eligible tablets from their site. You can view a copy from 4/5/15 on the Internet Archive. T-Mobile is now saying that any tablet that works on the T-Mobile network is eligible for free data for life. However users of many non-T-Mobile tablets are receiving a message that their device is not eligible when they try to sign up for FDFL. If this happens to you see this thread on T-Mobile's users forums for help in getting your device activated: Official Free Data for Life discussion: Can you add my device? | T-Mobile Support.
Source: T-Mobile via Phonenews