Comment 24 for bug 226753

Gene, "modprobe" is only a shortcut to "insmod", but apparently needs updates "deps" (what depmod generates).
I've missed the "sudo" in the commands I've given you to try, sorry for the confusion.
In fact, not "insmod" does survive the reboot, but now "modprobe vboxdrv" (as root in the init script) works for you.
I've no idea though, why the depmod cache wasn't uptodate before.

Bug 202542 appears to be related.