This was what the bulb looked like after unpacking...
Twisting the bulb did pull apart into two sections, but that didn't give access to the internal circuitry. DON'T DO THIS to the bulb!
Instead the bulb needed to be dismantled in a different way. By holding the plastic part of the bulb in one hand and holding the metal part in the other, these two parts could be pulled apart along the length of the bulb's body. The plastic section popped out without too much hassle.
The bulb had an antenna that stuck out so I was careful not to bend that. The plastic cover had a ridge on it which keep it in place. This ridge should be easy to get it back on when I'm going to be finished with hacking this bulb.
The internal circuitry was now accessible. The +3.3V, TX, RX, and GND pads were clearly indicated. These needed some wires soldered onto them so they could be connected to an FTDI. I've not done that yet.
I'll have more on the firmware flashing later. If you're looking to purchase this bulb, you can buy it online here: Sonoff B1: Dimmable E27 LED Lamp RGB Color Light Bulb.
Update: To see how to flash the firmware read this article - Hacking the Sonoff B1 WiFi LED bulb to run custom firmware.