The X200 model supported in particular right now is the 7458CY9, which is an older X200 variant. This X200 model has a 2.4GHz Core 2 Duo processor with 4GB of RAM and 160GB HDD. The X200S and X200T are also listed as being supported.
The X200 support hit mainline Coreboot today with this Git commit. According to the Coreboot Wiki, most of the X200 components should be working except for the dock, digitizer, and the need to be using binary blobs for the VGA option ROM, CPU microcode, and ME/EC.