I have installed a new system with FC 16 64-bit and in general all is working really well. I have also installed VirtualBox 4.1.8 from their website (as opposed to the "OSE" version from the Fedora repos). When I try and create a new VM it creates the new machine OK, but when going to the "First Run" wizard to really install the OS (in this case CentOS 6, but I have tried others with no success), the machine completely crashes (like you pressed the reset button) after specifying the disk layout.
uname gives - Linux fvb 3.2.7-1.fc16.x86_64 #1 SMP
Hardware details: Asus P8B WS mobo, Xeon E3-1270, 16GB non-ECC memory, SSD system disk, 500GB hard disk. (/home is on the SSD) A check for newer bios seems to indicate there are none. I see nothing about VT-x in the Bios setup, just VT-d. Anything realted to virtualization is enabled. A check of /proc/cpuinfo shows an empty file (??).
VBox has set VT-x and Nested Pages automatically when it created the machine.
From a historical perspective I have another machine running FC15, VBox 4.1.8, Quad Core CPU, and have had no trouble there building many different machines - CentOSs, Linux Mints, Windows XP, even Android for fun, no problems.
Thanks in advance for any suggestions.