I know you guys like VirtualBox, but I'm a vmware kinda guy. Here is my first contribution to the project. There are 2 free (as in beer) vmware editions -- vmware player and vmware server. What most developers want is VMWare Workstation, but that's $180. VMWare server is really too complicated and not well suited to the single developer -- but it's free.
Along comes a website that creates vmware machines without you needing vmware. http://www.easyvmx.com/ I created a vmware appliance with 256mg ram, settings for Ubuntu, bridged network, and 4.7gig ram (so it will fit on a dvd). Fortunately we are talking about just a couple files that zip down to 5k. That's right, "k".
Then I edited the .vmx file and put in the name of the Joomla Turnkey iso. If you have vmware player (I have workstation) installed, then you just put the iso in the same directory you unzip these files into....double click on the .vmx file and up pops vmware and the iso is booted. You have the option to run the live cd or install. That's it.
The way you would use this it to create a zip file for each of your appliances with the ISO file name already put into the .vmx file. That way the instructions would be merely to unzip the file in the same directory as the iso image and double click the .vmx file.
Or not. You can have just one zip file and give people instructions to edit the iso name in the .vmx file.
Hope you find this useful.