After revision 283678, which changed sys/dev/acpi_support/acpi_ibm.c, suspending with Fn+F4 no longer works on a Lenovo X220.
If the snippet below is removed from acpi_ibm_attach(), suspend works again.
/* Enable events by default. */
acpi_ibm_sysctl_set(sc, ACPI_IBM_METHOD_EVENTS, 1);
For anyone affected by this, sysctl dev.acpi_ibm.0.events=0 undoes those lines and gives the old behaviour.
NB: 283678 was by rpaulo, but it was 15 months ago.