However you can pick up your VMWare .vdk files and use them in VirtualBox. Easy..
Fire up VirtualBox. Go into the Virtual Media Manager and choose add and find your .vdk file.
Create a new machine and go through the wizard and choose the VMWare disk on the way. Here's the key bit, if you have a windows guest make sure you turn on Enable IO APIC in the System tab of the settings or your windows guest won't start. "Real" OSes seem to be OK ie my CentOS VM just worked.. The other sticky one is sometimes you need to go into the Storage tab and change the IDE controller type to PIIX3 for older windows guests. It's also worth giving the guests 32MB video memory or so and turning on 2D and 3D acceleration (through only for windows guests)
Otherwise your VMWare guest should just work.