VirtualBox is good and works well for grid tests, I expect it to be a fairly common solution. Same IP is not a problem as you make it work with port forwarding on your desktop. For example you should manage to build a grid like the following:
- -role hub on your Windows host on port 4444
- -role node on an Ubuntu VM with port forward 5556 in VirtualBox, for Firefox
- -role node on an OSX VM with port forward 5557 in VirtualBox, for Safari
- -role node on a Windows VM with port forward 5558 in VirtualBox, for IE and Chrome
- node with socat+flynnid.py on an Android emulator with port forward 5559
- maybe also iPhone Simulator from inside OSX VM (I don't remember if I made it work with grid)