I wanted to Import/Use a .vmdk hard disk image file from VMWare.  Generally you can just "point" VirtualBox to it and use it and it will work but I found an exception.

One of my Centos 4.4 x64 images wouldn't boot.  I had two copies, an older one and the newer one.  The older one booted as normal (once I changed the VirtualBox driver to IDE from SATA).  The newer one stopped at the "GRUB loading" message no matter what I tried.

As far as I can tell the size of the image was being reported incorrectly and perhaps this is a fallback/failure of Virtualbox to account for a situation with virtual disk files. 

The solution to boot this .vmdk was to convert using QEMU

qemu-img convert /location/of/your.vmdk your.bin

Then use VBoxManage to convert the QEMU image back to a native VirtualBox Image

VBoxManage convertdd your.bin your.vdi

After doing the above I was able to make the VMWare image boot.

I imagine this has happened to a lot of people and makes them panic/abandon the idea of using their VMDK/VMWare hard disk image with VirtualBox.

I also believe that the majority of them work fine.

To make things more scary, even if you boot using a LiveCD and have the .vmdk loaded, you won't be able to access the partitions unless you've done the conversion and this is what almost made me abandon the idea.  I thought I'd give the conversion a chance and was actually surprised that it worked since my other .vmdk images worked without modification and this one di dnot.