Ok, so it appears that there's no hardware issue connecting the x220 keyboard on the x230.
As expected, there are issues with the mapping, as reported in this review:
I managed to put the X220 old style keyboard and palm rest onto the X230. It fits perfectly but before you run out to buy a X220 keyboard in anticipation of getting a X230, the keys are mapped in the BIOS and exchanging them yields some strange results. While the letter characters work normally, the Delete key moves to the Page Up key and controlling the screen brightness is done using Fn + F7 or F8, like it is on the X230. Lenovo has moved some other function keys too. The audio controls have been moved to the function key row and the ThinkLight key, which also controls the backlight, has been moved to the Space Bar.
Source: http://www.laptopreviews.com/lenovo-thi ... ew-2012-05
I'm wondering how hard it is to modify the BIOS of a Thinkpad. I recall reading that custom BIOS for older TP had been made available, in the past.
I know it may be a bit soon to create such topic, but we may as well centralise informations as they come.