Due to the high usage of this guide and the lack of comfort in Gist's commenting area, I decided to make a blog post out of this which you can find here:
http://blog.frd.mn/install-os-x-10-10-yosemite-in-virtualbox/
An somewhat easier way with a bash script and step by step explanation can be found here: http://sqar.blogspot.de/2014/10/installing-yosemite-in-virtualbox.html
https://www.youtube.com/watch?v=tafqhSUfKnY points to: https://drive.google.com/folderview?id=0B9oZd81xqtTOckJOT25uTzVfT3c&usp=sharing The file Key.text made everything work for me under GNU/Linux+VBox 4.3.18!
VBoxManage.exe modifyvm "Yosemite 10.10 Retail" --cpuidset 00000001 000306a9 04100800 7fbae3ff bfebfbff VBoxManage setextradata "Yosemite 10.10 Retail" "VBoxInternal/Devices/efi/0/Config/DmiSystemProduct" "MacBookPro11,3" VBoxManage setextradata "Yosemite 10.10 Retail" "VBoxInternal/Devices/efi/0/Config/DmiSystemVersion" "1.0" VBoxManage setextradata "Yosemite 10.10 Retail" "VBoxInternal/Devices/efi/0/Config/DmiBoardProduct" "Iloveapple" VBoxManage setextradata "Yosemite 10.10 Retail" "VBoxInternal/Devices/smc/0/Config/DeviceKey" "ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc" VBoxManage setextradata "Yosemite 10.10 Retail" "VBoxInternal/Devices/smc/0/Config/GetKeyFromRealSMC" 1