Looking at various logs, I discovered:
:: whot@yabbi:~> dmesg | grep 56a [ 2.291841] usb 2-1.5: New USB device found, idVendor=056a, idProduct=0094Wacom's vendor ID is 056a, but the tablet in the x220 is a 0xE6, not a 0x94 (the latter of which doesn't exist). With that info, googling for a solution was a bit easier. As suggested in the thread on linuxwacom-devel a power off did indeed fix the issue for me.
:: whot@yabbi:~> dmesg | grep 56a [ 2.291841] usb 2-1.5: New USB device found, idVendor=056a, idProduct=00e6It turns out this is an issue with kernel 3.5.3 and 3.5.4, but it is fixed in 3.5.5. kernel bug. As Ping states:
The root cause is somewhere in the kernel tablet is set to a wrong mode. Once it is in that mode, a reboot won't reset it to normal. We have to power it off and wait a few seconds for all bits to reset. That's why I tell people to even unplug the power cable since bits may stay as long as they get power.
update 20/10/12: list kernel versions affected, add Ping's quote