Since Mavericks, OS X started checking signatures of kernel extensions. However unlike in Mavericks, loading unsigned code to kernel is disabled in Yosemite (in Mavericks only a warning, “Kernel extension is not from an identified developer” was displayed). It is required to sign kernel extensions using the Developer ID obtained after enrolling in Mac Developer program. With Yosemite still in beta, it is hard to find signed kernel extensions other than the ones officially released by Apple.
You will get this error when you try to load unsigned kext:
Diagnostics for [unsigned kext]:
Code Signing Failure: not code signed
ERROR: invalid signature for [unsigned kext], will not load
This being said, it is possible to sign all unsigned kext by doing this:
sudo nvram boot-args="kext-dev-mode=1"
Restart to get all the kext running!