After creating this topic, it looks like I have the same problem where my dongle is not supported in the RC (but it was supported in Alpha 4). My dongle is the Asus N-10 adapter and dmesg gives me this as the product/manufacturer details: Realtek 802.11n WLAN Adapter.
Someone had mentioned a problem here previously with that same dongle, but it was working just fine for me on Alpha 4. Now that I’ve upgraded to the RC it is no longer working.