I looked into that category (of small and lightweight laptops, for travel) earlier this year, without the coreboot requirement. I ended up with a Panasonic Let's Note SZ6-CF. Also cheap - imported from Japan via eBay - I think it is better than the X200 series in almost every way, newer, faster, lighter. It might also have a better display than the default of the thinkpads. Only drawback: soldered memory (a crime against the longevity of those machines).