4J

This is a very short summary of how I did my conversion. Perhaps it can be useful to some one else.

Preparation

1. Follow instructions from the VirtualBox site to add the Ubuntu repositories. Also add the GPG key as per instructions on the same page.
2. Run the following command:

$ sudo apt-get update
$ sudo apt-get install qemu virtualbox-3.0 dkms

Conversion

I had my vmware disk image split in 2GB parts, so firts we need to convert them into a single image (as per these instructions):

$ sudo vmware-vdiskmanager -r multipart.vmdk -t 0 flattened.vmdk
$ sudo qemu-img flattened.vmdk -O raw win_newdisk.bin
$ sudo VBoxManage convertdd newdisk.bin newdisk.vdi

Final Touches

You can now define a new VirtualBox machine and use the converted disk. After boot-up, un-install vmware-tools (if it was installed).

You should be good to go.