Snow changed the location of the caches. Now Extensions.mkext is in /System/Library/Cache, but I'm not sure if it will work just excluding the .mkext. Try boot with -v -f, with me always work.
Another question: For users DG31PR NEVER flash the BIOS, the USB stops working. USB only works with the BIOS version PRG3110H.86A.0052.2008.0612.1910.
I don't have that bios and my USB is working. I have the xxx.86A.0066.xxxx..
I have to have three extensions.mkext: in /extra, in /system/library and in /system/library/caches/com.cache.kext.com/startup. If i don't, most times my usb mouse doesn't work. And I have to regenerate them by hand with kextcache each time I install some kext or some installation that installs kexts