Thinkpad X31 Suspend to Disk

Hi I'm running a recent -CURRENT but I've had this problem for quite
some time now. I'm 100% certain my notebook has S4 BIOS support, and
everything is set up correctly for it to work (created a dedicated
hibernation partition and prepared it with tphdisk). At the loader
prompt if I press Fn-F12 I get the turquoise Phoenix NoteBIOS screen
and the system will actually suspend to disk and resume successfully.
However it doesn't work at all after I've fully booted, after issuing
acpiconf -s 4 the screen will blank and the indicator light will flash
for two or three seconds and then it shuts down without suspending to
disk. The sysctl hw.acpi.s4bios is 0 even though I clearly have BIOS
level S4 support. S3 level suspend works as advertised, and it's what
I've been using for awhile now but I would definitely appreciate any
help getting S4 sleep to work.

William Lam