The WRT1900AC takes the WRT54G and updates it from 802.11g to 802.11ac while doubling the number of antennas, is relying upon a 1.2GHz dual-core ARM SoC, and is adding in eSATA and USB connectivity for the router.
The WRT1900AC will still be open-source friendly with Linksys pledging to work with the OpenWRT development community. Linksys has said they will supply hardware and SDK/API documentation to these open-source developers.
If you're hoping this open-source router would be cheap, sadly it is not, like most other 802.11ac WiFi routers. The retail price once launched on the Linksys WRT1900AC is expected to be $299 USD. This WRT54G successor comes less than one year after Belkin acquired the Linksys division from Cisco.