NOTE: Refer to my legacy VirtualBox 3.1 + iSCSI on Debian Lenny mini-HOWTO, its time to update as VirtualBox 4.0.2 + Debian Squeeze.
VirtualBox 4.0.2 coming with feature "Teleportation" (aka live migration) since Virtualbox 3.1. With live migration you can port a running client from one host to another, without any service down time. It is for sure a enterprise-ready killer function (well... even KVM and QEMU already support that for some days before...). Therefore the next question should be: any pre-requirement for using VirtualBox's live migration? According to official user manual:
The two virtual machines on the source and the target must share the same storage (hard disks as well as floppy and CD/DVD images). This means that they either use the same iSCSI targets or that the storage resides somewhere on the network and both hosts have access to it via NFS or SMB/CIFS.
IMHO, iSCSI should be the best choice in this case: it is simple in architecture and faster when compare with using NFS/SMB/CIFS, it is native and simply supported by some SOHO SAN/NAS, and you may even share an entire hard disk to your virtual client machine as like as normal physical disk without abstract with NFS/SMB/CIFS (personally, this is my original and main target for using iSCSI).
Any drawback? Yes, for sure: you will not able to work with snapshot. This is because snapshot only work with *.vdi or *.vmdk
This mini-HOWTO with guide you though the installation of iSCSI target on Debian Squeeze, some demo setup for sharing your physical hard disk or a single raw file image with iSCSI, and howto setup an iSCSI hard disk within VirtualBox 4.0.2.