The nation’s #2 carrier, AT&T Mobility, today announced shared data plans, in response to Verizon’s shared data option which became available June 28. And just like the Big Red carrier’s Share Everything plans, new plans from AT&T allow new and existing subscribers to share cellular data across smartphones, tablets and other devices, plus get unlimited talk and text.

According to a media release, current customers, including business users, won’t be required to switch to the new plans. Should they decide to switch, they can do so without a contract extension. More importantly, AT&T says there will be no changes to its device upgrade policy…

This is in contrast to Verizon which is imposing its shared plans on everyone. Shared data at AT&T starts at $45 a month per smartphone plus an additional $40 for 1GB of data. You can choose your data allotment in 1/4/6/10/15/20GB tiers costing $40/$70/$90/$120/$160/$200.

The larger the data bucket you choose, the less you pay per gigabyte and the less you pay for each smartphone added to the shared plan. Adding a tablet to your plan will set you back an additional $10, or $20 in the case of a computer. Another smartphone with unlimited voice minutes and text messages will run your an additional $30 a month.

But don’t just think smartphones and tablets, says AT&T Mobility’s marketing chief David Christopher:

Today we think of people’s smartphones and tablets sharing a bucket of data. But in the future we’ll see health care monitors, connected cars, security systems and other devices in the home all connected to the mobile Internet. Our Mobile Share plans are simple, easy and a great value for individuals or families with multiple mobile Internet devices.

Here’s an overview of AT&T’s shared wireless data plans, features and prices.

You can add up to ten devices to your shared plan, one of which must be a smartphone. Basic math tells us that having an iPhone and iPad share data on AT&T’s network would cost $120 a month for four gigabytes of data: $70 + $40 for the iPhone and $10 for the iPad.

Want ten gigs? The cost goes up to $180 a month. A PC/Mac with ten gigs of data? That’s a total of $180 a month, paid to AT&T each month.

Have you guys already made calculations? Is this new shared data option going to reduce your monthly bill?