Just started using Sun's VirtualBox. VMware, Parallels ought to shaking in their closed sourced boots. It was easier to install and configure than Parallels and VMware and is FREE (as in beer). Sun even released a open source version. Parallels is such a bane running on Mac OS X, but VirtualBox seems to run smoother. I was even able to PXE boot a guest OS without using a floppy image like I have to do with my older version of Parallels.
A couple of things make VirtualBox different than other hypervisors. First, at least on the mac version of VirutalBox (VB), the networking differs in the following ways. This is in the manual too.
1. The network between the host OS (the OS that is native to the machine and that VB is installed on) and the guest OS (the OS that you are installing inside VB) is implemented in user mode. What this means is that VB does not reach into the host OS kernel and try to glob onto the network stack, Instead, VB just takes network traffic from its guest OS's and sends it the same way any other app on the host does. VB decided to do this using NAT by default. So VB just needs to present one logical "port" to the host OS for the network. So you can think of the VB hypervisor as a router, just like the one you may use at home to have multiple computers behind a cable/DSL modem. So the hypervisor has the ip of the host OS and it masquerades all its guest OS's as this ip. Pretty easy, makes sense, but definitely less convenient.
2. You can "bridge" the network connections to your guest OS's too. This is more common in paravirualization and virtualization. There are directions to do this in the manual. There appears to be no way to do this for VB for Mac OS X (using VB on Mac OS X host OS). Oh well, maybe in later versions. You can still port forward while you use NAT. I would not need this feature at this time anyway.
3. I installed VB on Mac OS X host OS and then PXE booted the debian etch installer and noted some things that can save you some troubleshooting time. First, you must make a TFTP directory and copy your debian installer files and pxelinux.0 image there. Also, and this tripped me up good, you CANNOT pxe boot into an image that has a space in it. For example, I named my VB image "warehouse image". So when pxelinux tried to go and find its configuration (by default: pxelinux.cfg/), VB passed it a directory prefix of "warehouse" WITHOUT the 'image" part. Bad news. I renamed the image to just "warehouse" and it worked. So here is the list of what to do to make VB PXE boot.
a. create an image in VB (be sure not to use white space in image name, use foo and NOT foo bar)
b. configure the boot options to boot from network
c. create a new directory called TFTP in ~/Library/VirtualBox/
d. on your mac, rename "pxelinux.0" to foo.pxe and copy it to ~/Library/VirtualBox/TFTP/
e. copy the entire directories of pxelinux.cfg and debian-installer to ~/Library/VirutalBox/TFTP/
f. the pxe system can be obtained from debian in a netboot file called netboot.tar.gz (this contains all the files you need to pxe boot into a debian installer).
A couple of things I noted AFTER debian was installed and I reboot into it. I reboot after everything was installed and since it was set to network boot before, it tried to network boot again. This time, it got an IP from the internal VB dhcp server (yes, the hypervisor runs an internal dhcp server), it then found the tftp (next server) and boot attempted to find a configuration inside pxelinux.cfg but COULD NOT because "TFTP server does not support the tsize option". OK so I let it sit and about 10 minutes later, it booted anyway! I have seen this tsize error before and it is well documented in the pxelinux FAQ's. They recommend using a TFTP server that supports the tsize option such as "tftpd-hpa", but why the VB tftp server worked before, but not after the debian install is a mystery. Installing an OS should not have anything to do with this tftp server.