Have a look at Lenovo's X series.
They are all limited to 8GB. According to [2] the new X250 is, like the new X1, also a single DIMM.
[1] - http://shop.lenovo.com/us/en/laptops/thinkpad/x-series/
[2] - http://www.anandtech.com/show/8822/lenovo-thinkpad-x250
X230 (Ivy Bridge, 2 years old) supports 16GB and has the good keyboard/trackpad.