virtualbox.org

Host: Windows Server 2008 R2 Enterprise 64-bit
Guest: Debian GNU/Linux with Linux 3.2.0-4-amd64 (64-bit obviously)

VM starts, GRUB loads, but after ramdisk loading it shows black screen with horizontal bar cursor in the upper left corner that doesn't blink.

*attached vbox log

Sounds to me like grub failed to mount the filesystem.

Have you tried chrooting, checking for errors and running mkinitcpio -p linux?