The Nexus 7 3G model is a fully unlocked tablet, supporting any GSM carrier SIM card for data. But when you bought a Nexus 7 3G until today, you would get an included AT&T SIM card for quick activation. Now, this does not mean that it is an AT&T device. There is no hardware branding and it is not locked to that carrier, but they included the SIM so you could get started faster.
Today, they are offering a T-Mobile version of the package. It still comes with an identical unlocked Nexus 7 3G, but it includes a T-Mo SIM instead of the AT&T one. It really isn’t a big deal, but if you plan on using this device on T-Mobile, it might make the setup process faster than going to a store. It’s nice that there are options, but I hope people don’t get confused and think that they are carrier specific tablets.
However, one thing AT&T offers that T-Mobile does not is a $100 subsidy when signing a two year contract. You can get that offer when activating the device.