I was having a confusing issue where after moving an ubuntu virtualbox virtual machine I was losing the virtual network adapters (i'm running one on NAT so the machine can access the internet via the host and one host-only so I can network between host and guest). After a bit of head scratching, I realised this was because the way I was "moving" a machine was by copying the hard disk image and creating a new machine. This means that the MAC addresses were getting reset. If I went into the virtualbox machine settings and changed these to the same mac addresses as were being used on the vm before it was moved, the network interfaces reappeared. I guess that's why I should be moving the machines with the config file too..

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