Hopefully someone will correct me on this, UPDATE: some one has corrected me on this, although I haven't tried what they suggest - see comments below) but I was disappointed to find I couldn't move a server I set up on VMware fusion on my macbook to vmware server (free version) running on win xp (only temporarily, don't panic). However, I understand it will run on the paid version and in vmware player. I'm also assuming that I won't be able to move VMware fusion machines to the free version of vmware server on linux, but haven't tried (send comments if you know otherwise!).
I actually moved from parallels to VMware fusion on the basis that I would be able to simply move VMs from my mac to a production server, but it looks like unless I cough up for the paid version of VMware server, this isn't going to happen - very frustrating. Time to investigate KVM