Modules fail to load with an invalid format error when built against binary sys-kernel/debian-sources kernel. After emerging kernel, updating /usr/src/linux, emerging virtualbox-modules and nvidia-drivers, and rebooting the resulting modules all fail to load with same invalid error.
vboxdrv for example :
Error inserting vboxdrv (/lib/modules/3.2.15-1/misc/vboxdrv.ko): Invalid module format.
Possibly related kernel message
May 20 21:11:01 [kernel] [ 39.610061] vboxdrv: no symbol version for module_layout
May 20 21:11:01 [kernel] [ 39.631311] vboxdrv: no symbol version for module_layout

Originally reported on forums here: http://forums.funtoo.org/viewtopic.php?id=1370