Shopify partnered with Themezilla, Themify, and Ultralinx for the launch. The themes and plugin are free and Shopify is promising user they can, “still manage all of your pages and posts in WordPress, but you’ll have Shopify to manage everything else: payments, secure checkout, shipping and fulfillment, inventory, and taxes.”
I know a lot of folks will be excited about this for simple eCommerce stores to integrate with WordPress sites. It doesn't require a custom domain, and instead utilizes a slide-out cart and a pop-up checkout form that redirect to checkout.shopify.com for the secure transaction. It appears that Shopify worked with WebDevStudios for the integration.
It was previously possible to use Shopify with WordPress, but it was a lot more difficult to integrate, according to SkyVerge and SellWithWP owner Beka Rice. Shopify also used to have an integration with WordPress.com, but that program has since ended (I believe around the time Automattic acquired WooCommerce — though it seems it was killed in part due to lack of usage as well).
Self-hosted eCommerce isn't for everybody. There are many challenges to consider. So for folks seeking simple stores and a hosted option, Shopify isn't the worst option, and the integration of the buy button for WordPress sites makes it so more of the shopping experience can be maintained on the WordPress site itself; and that's a good thing I think.