802.11ac will support fallback to older Wi-Fi standards to enable backwards compatibility. Some devices may only be capable of 5GHz, which means they'd only be able to revert to 802.11n. However, many devices will most likely still be dual-band, and be capable of switching to 2.4GHz and connecting with 802.11b or 802.11g if necessary.
Keep in mind that the 802.11ac specification is not yet finalized. Equipment and devices that come out this year will be based on the draft, and there is no guarantee they'll work with other proprietary draft 802.11ac equipment, or that they'll be fully compatible with the official 802.11ac standard once it's finalized.